IN the section on Anatomy, we learned that the heart consists of four muscular chambers, and we considered the main points in the anatomy of the circulation as a whole. Here it is our business to discuss the mechanisms by which the circulation is kept going and under what circumstances it varies.
If we cut out the heart from an animal immediately after death, we shall find that it will continue to beat rhythmically for a short period of its own accord. If we now supply it, through the openings of its vessels, with a nutrient fluid, we can keep it going for a longer period, and we can study it as simply as we could study any other purely mechanical machine.
We shall find that the heart of itself has an inherent power
of beating rhythmically and that the direction of the blood flow through the heart is always the same and is determined by the valves. The first thing that happens is that the auricles fill from the veins and then contract, driving the blood through the auriculo-ventricular valves into the ventricles. Both auricles contract simultaneously. One-tenth of a second later the ventricles contract, again simultaneously, and drive the blood into the arteries. Regurgitation of blood, as we have seen, is prevented by the valves.
LAWS THAT THE HEART MUST OBEY
THERE are two ‘laws ‘which govern the working of the heart as a machine and which are of the greatest importance and interest. The first is that each muscular fibre of the heart always contracts to the very best of its ability at any one moment. A fibre can give only one kind of contraction at any moment, namely the maximal. A fibre contracts completely or not at all. This is known as the ‘All or None Law.’ The maximal contraction will vary from time to time according to circumstances, but each contracting fibre is always bound to do its best under the circumstances existing at the moment.
The second law is known as ‘Starling’s Law of the Heart,’ and states that within certain limits the longer a muscle fibre becomes, owing to stretching, the better and more forcibly it will contract. The best that a fibre can do is increased if the fibre is lengthened.
The vast importance of this is that, if a larger than usual amount of blood comes to the heart via the veins, the heart will be unusually distended with blood and its muscle fibres will be lengthened. They will therefore be capable of more vigorous contraction, and the heart will consequently be able to pump out the extra amount of blood without becoming dilated or actually bursting.
It is a defence mechanism, preventing the heart from being disabled and automatically ensuring that the heart responds at once to the work required of it. In exactly the same way if the pressure in the arteries is increased for any reason, as in exercise, the heart must do more work in pumping out blood against a greater resistance. Temporarily blood is dammed back into the heart, but immediately the fibres become lengthened owing to the distension by the blood dammed back, and they contract more forcibly enabling the heart to cope with the increase in work required of it.
THE NERVES THAT CONTROL OUR HEART-BEATS IN the wall of the right auricle is a collection of nerve cells which start the contractions of the heart. From this point the contraction spreads over the heart like a wave. It is by virtue of this collection of cells that the heart is enabled to beat when it is cut out of the body. These nerve cells are known as the sino-auriculo node, and in the body they are controlled by two nerves coming from the central nervous system—one, the vagus, from the brain; the other, the sympathetic, which comes eventually from the spinal cord.
These nerves influence the rate of the heart. This can be proved if the nerves are cut one by one. If the vagus is cut, the heart will beat more rapidly, for its slowing influence is removed. If the cut end which leads to the heart is stimulated by an electric current, the heart will beat more slowly. If the sympathetic is cut, the heart will beat more slowly, and if its cut end is stimulated, the heart will beat faster. The vagus, therefore, slows the heart and the sympathetic quickens it.
In the walls of the great veins entering the heart are certain nerve fibres which are sensitive to stretching when the veins are dilated by a large volume of blood. These nerves collect themselves into a bundle of fibres, which pass to the central nervous system and form connection with the vagus and sympathetic. They subserve a reflex known as Bainbridge’s Reflex, and their function is to drive the heart faster in order to cope with an extra amount of blood coming to it. When the veins are engorged, these nerves are stretched and impulses pass to the nervous system which inhibit the vagus and stimulate the sympathetic so that the heart beats faster.
On the other side of the heart, in the wall of the aorta, are some nerve fibres which join to form the depressor nerve which subserves the depressor reflex. If the pressure in the aorta is increased, these nerve fibres are stretched and impulses pass up them to the nervous system, where the vagus is stimulated and the sympathetic inhibited so that the heart is slowed. This, of course, is a defensive mechanism to prevent the blood pressure from becoming too high. Both these reflexes act purely automatically, of course, and we know nothing about it when they are brought into play, for their activity never comes into consciousness.
HOW THE BLOOD IS SENT BACK TO THE HEART AT first sight it would seem difficult to understand why blood returns to the heart from the capillaries without anything to drive it. The heart itself drives the blood to the capillaries, but there is no heart to drive it back. There are three things that assist the return by the veins, the first being the fact that there are valves interposed in the course of the veins, preventing the blood flowing backwards—away from the heart. This can be demonstrated quite clearly by choosing any vein which stands out sharply in the arm and running a finger firmly down it towards the hand, finally compressing the vein with the finger so that no further blood can enter it. The blood will flow backwards from above until a valve is encountered, where it will be held up. The vein below the valve, and between it and the finger, will remain quite empty so long as the pressure is kept up.
The flow of blood back to the heart is greatly assisted by the contraction of the muscles which under normal circumstances is continually taking place. The muscles squeeze the blood along the veins and help to counteract the influence of gravity. The third factor of great importance is the negative pressure—I.e. pressure less than atmospheric—which exists in the thorax and which is greatly increased when we inspire. This negative pressure which surrounds the great veins entering the heart is constantly tending to suck blood into them, thus returning it to the heart.
WHAT HAPPENS BETWEEN HEART-BEATS
THE pressure which the blood exerts in the arteries is, of course, dependent on the amount of blood which the heart pumps out on each beat. If the arteries were rigid tubes, this pressure would fall and rise with every beat of the heart, or would fall almost to zero between each beat when no blood was leaving the heart or entering the arteries. Nature has guarded against such an untoward result by making the arteries elastic, so that they expand when blood is forced into them and store up the blood.
When the heart finishes its beat and is resting in preparation for the next, the arteries rebound by their elasticity, so that the pressure is kept within reasonable limits and blood is forced into the capillaries in a continuous stream, instead of in an intermittent one. In old age the arteries become dc-
generate and hardened and lose their elasticity, so that the blood pressure becomes higher and fluctuates more when the heart beats.
THE COMPLICATED PROCESS OF RUNNING A RACE w
E have seen briefly how the heart works by itself and how it works when controlled by its nerves. In order to see it in action in relation to the body as a whole, let us see what is the effect upon it of exercise, say the running of a strenuous hundred-yards race. The muscles all over the body, but especially those in the legs, are working much harder than normal, so that they will require a greatly increased blood supply in order to enable them to work efficiently. This extra blood must be supplied by the heart which pumps blood round the circulation more rapidly.
Because the muscles are working and contracting, they will squeeze more blood back to the heart, and this effect will be helped by the increasing rate and depth of breathing, which will suck the blood back into the thorax. Respiration is increased, of course, so that the blood may become more oxygenated and therefore be of more use to the hard-working muscles. The increasing venous return, both because it elongates the muscle fibres of the heart and because it sets the Bainbridge reflex in operation, results in the heart doing more work and throwing out more blood into the circulation at each beat.
Meanwhile the suprarenal glands secrete adrenalin into the blood stream, and this substance increases the force of the heart beat, adding its support to all the other mechanisms. Adrenalin, however, has quite a different action as well, for it has the peculiar property of making the blood vessels of the structures inside the abdomen contract and squeeze out their blood, which then becomes available for a more important purpose. Adrenalin ensures that no blood is wasted in supplying organs that do not need it at the moment. It sends it to organs whose need is greater.
Such are the provisions that Nature makes to enable us to work without undue strain being put upon our hearts or muscles. Such a system works to perfection so long as the strain is not too great. The heart, however, being a vital organ, must be protected from overstrain, and it will always happen that the muscles will become too tired to work before any dangerous strain is placed on the heart. Work will always
cease and allow of a breathing space before the vital structures are endangered.
It follows from this that a person with a perfectly healthy heart cannot possibly strain it by doing too much exercise, whether by road-breaking or by playing a hard game of football. It is only in those cases where the heart is already diseased and where the staying power of the muscles exceeds that of the heart that any damage can be done. The muscles may then outwear the heart and subject an already diseased organ perhaps to breaking strain. This is the reason why people who really suffer from heart disease must be extremely careful in the exercise they take. But the heart is far less easily damaged than is generally supposed.
THE testis, the organ which produces the male germ cells or spermatozoa, elaborates also a hormone which is responsible for the secondary sexual characteristics that appear in man. If deprived of this organ, man fails to grow hair upon his face and body, and the voice remains high-pitched and ‘unbroken,’ owing to under-devc/lopment of the larynx. Eunuchs, too, are invariably fat.
Voronoff has claimed that many of the manifestations of
old age are due to the fact that the testis runs out of secretion and fails to keep the body in good repair. He grafted the testes of monkeys into patients who were growing old and claims that they became rejuvenated. Whether his results are as good as popular opinion would lead us to suppose is uncertain, but it would seem to be a dangerous experiment, for it is, as it were, driving too hard a dying horse. The last years of life may be made more exciting and the patient may feel younger, but the machine will probably run down more rapidly and life come to an end sooner and in a more spectacular manner.
It must not be supposed that the ductless glands work separately and independently of one another. Each affects the others, and any upset in one will lead to corresponding upsets in the rest. A balance is normally effected between all the hormones circulating in the blood, and on this balance our well-being depends. Some people place disturbances of the ductless glands as the cause of many of our obscure conditions of ill-health, others say that they are responsible for all the differences in personalities and psychical qualities that exist between each one of us. However much truth there is in these theories, one thing is certain and that is that we know very little of their relationships one to another, that much will be found out in the next few years, and that meanwhile we should be wary in upsetting the balance by dosing people with products of the ductless glands without adequate cause.
WE have seen how the ordinary glands of the body produce their secretions and how the latter pass down particular ducts to the sites where they act. We have now to deal with those glands which have no duct and pass their secretions direct into the blood stream. These glands are therefore known as ductless glands.
In those humble forms of life, in which the organism contains only one cell, any movement or reaction which the cell undergoes must be due to physical or chemical stimulation by its environment. Single-celled organisms will be attracted or repelled by heat or cold or by chemical substances present in solution around them. In those animals, which contain not one but many cells, a similar response takes place, for certain glands throw out into the blood stream substances which act as messengers, producing reactions in those cells with which the blood comes in contact. These chemical messengers are known as hormones.
THE THYROID GLAND: SPEED REGULATOR OF OUR LIVES PERHAPS the best known of the ductless glands is the thyroid, which lies in front of the windpipe or trachea in the neck. This gland is composed of a number of more or less spherical
bulbs or alveoli. They are like flasks but there is no opening. The interior of the alveolus is filled with a transparent mass of material called colloid, which is secreted by cubical cells lining the alveolus. Between the alveoli are numerous blood vessels into which the secretion is absorbed to be distributed all over the body. The active principle of the thyroid gland is a substance called thyroxine which has been completely investigated chemically, whose formula is known, and which has been synthesised in the laboratory. It is the only ductless gland secretion which exerts any effect when taken by the mouth, for it is unaffected by digestion. All the others are completely destroyed by the processes of digestion so that they must be taken by injection to exert any effect.
The role played by the thyroid has been worked out by studying the effects of the removal of the gland from animals and by injecting extracts of the glands into animals from which it has been removed. Also, certain individuals, known as cretins, have the misfortune to be born with insufficient thyroid secretion, while others suffer from a disease known as exophthalmic goitre in which the gland secretes excessively.
The cretin presents a picture of under-development both
mental and physical. Stunted, pot-bellied and ugly, a cretin of twenty may have the appearance and intellect of a child of four. If treated sufficiently early, by feeding them with thyroid tablets, these creatures may be turned into intelligent and useful members of society.
WHEN THE FIRES OF LIFE BURN LOW: A STRANGE DISEASE IN adult life, if the thyroid undergoes regression, a condition known as myxcedema is produced. The unfortunate person becomes slow and dull of intellect and so forgetful that it is quite impossible to cure his condition by giving him a bottle of thyroid tablets to take home, for he will forget to take them when relapse occurs. The skin becomes coarse and dry and the hair falls out. The whole aspect of the individual changes and he sinks into a purely vegetative existence. The temperature and pulse rate fall, and he will complain bitterly of cold, owing to the lowered rate at which the fires of metabolism are burning. The appetite goes and there is little desire for food, and with it falls the amount of waste products secreted in the urine. If the basal metabolic rate, which is the measure of the warmth and energy given out by an individual at rest, is measured it will be found to be enormously below par. This condition is completely curable by feeding the patient with thyroid tablets, but they must be administered for the rest of his life and never given up, and it must be seen that he takes them regularly.
THE PATIENT WHO LIVES TOO FAST
A T the opposite end of the scale is Graves’s disease, in which ifjLthe thyroid exhibits a high degree of over-activity. The condition is often precipitated by a nervous shock, and the patient presents a picture of great excitability. He is exceedingly restless and always fidgeting, worrying incessantly about trivial matters of the least consequence. Sweating is profuse and the heart beats at a greatly increased rate. The temperature is raised and, owing to the rapid rate at which food and body tissues are burnt up, the patient becomes very thin. The basal metabolic rate will be found to be greatly increased. The surest method of curing this condition is to remove the greater part of the enlarged thyroid gland by an operation, the results of which are sometimes almost miraculous.
It is therefore clear that the thyroid regulates the rate at which we live. It is like the tap on the side of a gas oven
which controls the temperature inside. In addition to this, it is responsible in part for growth and mental development.
Some years ago, when the parathyroid glands were insufficiently understood, it was noted that patients who had their thyroid gland removed for Graves’s disease sometimes went into a condition known as tetany. Their nervous systems became enormously over-excitable and, at the slightest provocation, their hands and feet would go into a painfui spasm. Later it became known that this was because some small glands, situated inside the thyroid but differing from it in function, had been removed with it. These were the parathyroid glands.
It was found in addition that the people who suffered from tetany had less chalk in their blood than normal people. In contrast to this, if a normal animal was given injections of an extract of the gland, it was found that the amount of chalk in its blood went up until eventually it collapsed and died of vomiting and diarrhoea. It will be quite obvious, therefore, that the parathyroid hormone regulates the amount of calcium or chalk in the blood. Exactly how this is done is not quite certain, but there is strong evidence to show that the hormone in some way makes the calcium leave the bones and enter the blood.
This suggestion is supported by the fact that people who have tumours of their parathyroids, causing them to secrete too much hormone, have a curious condition of their bones which, because they have too little chalk in them, are rarefied and brittle and break under the slightest strain. When the parathyroid glands are removed, the bones return to their normal condition, showing clearly that the glands were the cause of all the trouble.
THE GLAND THAT DEFENDS US IN EMERGENCY LYING on the top of the kidney on each side of the body is a ygland known as the suprarenal. It consists of two quite separate parts which have nothing whatever to do with one another. There is an outer shell or cortex of a yellow colour and an inner dark-brown mass known as the medulla. The cortex is essential to life, for if it is diseased or removed the animal becomes pigmented and its blood pressure falls until eventually it dies. Sometimes when the cortex is over-active, the individual reverts apparently to the opposite sex. For example, a woman will develop a coarse beard all over her
Face, like a man. This is often the cause of the so-called bearded women sometimes seen in circuses.
The medulla, however, is quite different, for it secretes a substance known as adrenalin, which is now well known and has actually been made in the laboratory. The action of adrenalin may be summarised by saying that it makes the animal alert and puts it in the best position to defend itself when attacked. Thus if adrenalin is injected into an animal, it makes all the blood vessels contract in those parts of the body which are not useful in fighting and drives the blood into more useful channels. The blood vessels of the intestines are emptied so that more blood can go to the brain and muscles. At the same time, adrenalin increases the force and rate at which the heart beats, so that more blood is available for rapidly-working organs. The effects of adrenalin are exactly similar to those produced by the sympathetic nervous system, for the suprarenal actually is a part of the sympathetic nervous system and is actuated by it. One other action of adrenalin is of interest, and this is its effect on the muscles which control the hairs. Each hair has a minute muscle attached to its base which, when it contracts, will make the hair stand on end. Both stimulation of the sympathetic nerve and the application of adrenalin will result in contraction of the muscle so that the hair is erected.
If this happens all over the body, as it does, for example, in the cat, the animal will be bristling with hairs all over and will appear much larger and more formidable to its enemies. The same action is seen in the hedgehog, only here it produces not only an apparent but also a real protection, for with the spines erected the animal presents an almost impregnable fortress. In man, goose-flesh is produced by some shocking or revolting sight, and excessive fear will result in our hair standing on end.
The proof that adrenalin is secreted under conditions of fear and can cause these effects is experimental and very interesting. A cat, under anaesthesia, had a long glass tube passed into a vein in its leg and running right up into the abdomen to a point where blood was returned into the inferior vena cava from the suprarenal glands. Blood from the supra-renals could thus be drawn off directly whenever desired. After a time, when the cat had recovered from the anaesthetic, it was frightened by a dog barking in the same room and some of the blood from the tube was drawn off. When this was used
for perfusing an isolated heart, the rate at which the heart was beating was increased. In addition, when this blood was injected into another cat, all the effects of adrenalin were produced.
THE GLAND THAT CONTROLS OUR GROWTH SITUATED at the base of the brain and hanging down from it is a gland known as the pituitary. It is divided by a septum into two parts, known as the anterior and posterior lobes, which have quite different functions. The anterior lobe is one of the main organs which control growth in young animals and keep it in check in older ones. If the pituitary is experimentally destroyed in young animals, they fail to grow and they also fail to exhibit sexual development—that is, the organs of sex remain in an infantile condition.
Overgrowth of the gland has been shown to give rise to gigantism, a condition in which all the bones of the body show a more or less harmonious overgrowth, with the production of a giant instead of an individual of normal proportions. In the same way, rats may be caused to grow to enormous sizes by giving them injections of the anterior lobe of the pituitary when they are young.
If the overgrowth of the gland occurs when the individual is already fully grown, it produces the curious condition known as acromegaly. Here only a few of the bones undergo unusual enlargement, these being the hands, which are large and spadelike, and the lower jaw, which becomes heavy and projects forwards unduly, so that the lower teeth come to lie in front of the upper when the jaws are closed. The face grows long and heavy, and the nose becomes very accentuated, the whole facial expression being changed to one of extreme ugliness such as is best described by the expression ‘lantern-jawed.’
The posterior lobe secretes several hormones, one of which makes the blood vessels contract and another which has a special influence on the muscle of the womb during labour. For this reason it is much used during childbirth when the womb contracts only feebly. This substance goes by the name of ‘pituitrin.’
THE GLAND THAT CONTROLS MENSTRUATION IN addition to producing the ova or egg cells, which we shall consider later, the ovary has several very interesting and important functions. In childhood it secretes a substance
known as cestrin, which is responsible for growth and develops the secondary sexual characteristics. In fact it makes a girl into a woman and is responsible for all the differences between men and women that appear at the age of puberty. At this age a woman commences to menstruate, that is, she bleeds every month from the womb. This process, which has caused so much trouble, is really a mechanism by which an egg cell, which is discharged from the ovary every month, may, if it becomes fertilised, find a new and clean home in which to develop. When the ovum does not become fertilised, the womb casts it out, as it were, and repairs itself in readiness for the next opportunity.
The ovary contains certain structures, known as follicles, which produce the ova each month. Before the ovum is ripe and is ready to be thrown out of the ovary, the follicle produces cestrin in fairly large amounts. The action of this substance is to prepare the inside of the womb for the reception of the fertilised egg. Later, when the ovum has been cast out of the ovary and is ready for fertilisation, the remains of the follicle produce a new substance known as lutein, which keeps the womb in good condition and assists the implantation of the ovum.
If the ovum is fertilised, it becomes imbedded in the womb and pregnancy ensues. The lutein secreted by the ovary helps in this process and prevents menstruation occurring. It also produces enlargement of the breasts, so that the infant can suckle when born. After eight months it begins to disappear and allows labour and the birth of the child to take place. If, however, the ovum is not fertilised, the follicle, which is still producing lutein and stopping menstruation, begins to disappear after ten days, so menstruation takes place in preparation for the next ovum which the ovary will discharge the following month.
THE object of digestion is to break down the constituents of the food into the simplest parts, so that the food is more readily absorbed. Also the proteins must be broken down first, so that they may be built up later into proteins characteristic of the animal that uses them. Mutton protein as such would be useless to man. It must be absorbed in pieces so that he can make his own peculiar protein for himself.
Digestion is accomplished by means of certain juices which break up the constituents of the food. These juices are secreted by glands. A typical gland is composed of a number of cells arranged to form a flask or retort, from the opening of which a duct leads. The cells have the power of taking substances from the blood and converting them into juices or secretions which are carried down the duct to the place where they are used. Each gland is supplied with nerves which control its working.
When food is taken into the mouth, it is broken up into small fragments and intimately mixed with the saliva which is secreted by certain glands in the neck. This saliva has the power of breaking down the starches and polysaccharides into disaccharides. The stimulus which makes the glands secrete is not only the presence of food in the mouth, but even the sight or smell of food. It is a well-known fact that the ‘mouth will water ‘even at the sight of some tasty morsel.
When the food is swallowed, it reaches the stomach, and here it comes under the powerful influence of the gastric juice, which breaks down the proteins into proteoses and peptones and has some small action on the sugars. The mechanism of
the secretion of gastric juice has been well shown by Pavlov’s experiments with dogs. He cut the gullet in the neck and brought each end to the surface, so that food swallowed by the dog did not go into the stomach but was discharged from the cut end. He also brought a small part of the stomach to the surface of the body, so that he could directly watch how much juice was secreted.
He found that the sight or smell of food, even if it was not given to the dog, would start the stomach secreting. If food was swallowed, the stomach would secrete, even if food did not reach it. Also if food was introduced into the stomach without the knowledge of the dog, as could easily be done, the stomach would secrete, but the amount of secretion depended on the character of the food. Meat was more potent in producing a flow than bread, for example. Here, laid before us, is the whole mechanism of glandular secretion. First comes the psychical secretion at the sight of food, then the mere presence of food in the stomach from its mechanical action, and thirdly the chemical stimulus from the nature of the food.
HOW THE FUEL IS BROKEN UP
AFTER the food leaves the stomach it enters the duodenum, which is the next part of the bowel. Into this open two ducts, one the bile duct from the liver and the other the pancreatic duct from the pancreas. The juice from the pancreas contains several ferments which act in conjunction with the bile. This, though it contains no ferments itself, is a powerful assistant to the pancreatic juice. Between them these two juices break up the remains of the proteins into polypeptides and amino acids, the fats into fatty acids and glycerine, and the sugars and starches into monosaccharides.
Some few of the food constituents will have escaped the breaking-down process, and these are acted upon by a juice, the succus entericus, which is secreted by the small intestine itself. The result is that the contents of the intestine now consist of a fluid mass containing amino acids, fatty acids, glycerine and monosaccharides in such a condition that they are ready for absorption.
HOW FOOD GETS INTO THE BLOOD
THE intestine is well supplied by blood vessels and lymphatics into which the products of digestion can be taken. The mechanism by which this is affected is by no means certain,
and the probability of its exact working being discovered is remote, for the process is an obscure and essentially vital one that would require a knowledge of the deepest secrets of the cells themselves. It may be stated that during absorption work is performed by the cells, for they have to work against a definite pressure. The contents of the intestine are more concentrated than the blood, and therefore water will tend to pass out of the tissues into the bowel.
This tendency to equalise concentration on both sides of a membrane is known as osmotic pressure, and the cells have to work against it in absorbing material into the blood. One thing, however, we do know, and that is that the finger-like processes known as villi, which cover the inside of the intestine, are contractile elements. Their function is to increase the effective area over which the absorption can take place and also to assist it mechanically.
A villus is a minute structure, like the frond of a sea-anemone, covered on the outside with absorbing cells and filled with soft, connective tissue. Running up the centre is a lymphatic vessel and blood vessels into which the products are absorbed. There is also a muscle which can increase and decrease the volume of the villus, so that suction on the contents of the bowel can be applied. This suction sucks the bowel contents through the cells and not through any opening. Under certain circumstances the villi can be seen contracting rhythmically under the microscope, proving the action. This is all that we know of the mechanism of absorption, and it is by no means the whole story.
The fats, which have been re-synthesised in the villi after their splitting in the bowel, are absorbed into the lymphatics, and later enter the blood stream. The sugars and amino acids are absorbed into branches of the portal vein, and pass from there to the liver, as we know from our anatomy.
THE LIVER : CHEMICAL FACTORY OF THE BODY w
HEN the sugars reach the liver, they are converted by the activity of the liver cells into glycogen, which, as we have mentioned before, is a complex starch made up of many monosaccharide molecules. The glycogen is stored in the liver and later, as required, split down again into glucose and passed into the circulation, where it is used directly by the tissues. The liver, thus, is a storehouse for sugar and prevents its too rapid use.
Of the amino acids absorbed, some are burnt up in the same way as the sugars are, in order to provide warmth and energy, and the liver plays a large part in this process. Before they can be burnt, they must be converted into sugars, and this action is performed by the liver, which removes the nitrogen from their molecules so that it can be excreted by the kidneys. Those amino acids which are not burnt up for energy escape the gauntlet of the liver and pass to the body cells in general, where they are built up into complex human proteins.
This is the part of our food which replaces the wear and tear of the body and is so essential for the well-being of an adult and the growth of a child. Wc require actually only sufficient protein in our diet to enable us to replace our wear and tear. Sugar and fat can supply the energy, but they can never be of use in this particular way. This, too, is why growing children require so much more protein than adults.
The fats also eventually reach the liver by way of the general blood stream. Some of them after absorption are taken up by the fat cells under the skin and in other parts of the body, and stored there until they are wanted for the production of energy. Before they can be burnt, however, they must pass through the liver, wliich changes them slightly and so makes them more readily combustible.
The liver is one of the important workshops in which sugar is converted into fat. That this is possible in the body is interesting, for it is the cause of fatness in many people to-day. It was for long considered that excessive eating of sugar could lead to obesity, and it was proved conclusively by feeding two pigs of the same litter, one with and one without sugar. The weight of fat in the pig fed with sugar greatly exceeded the fat in the body of the one who had gone without. The majority of people to-day who are unduly fat could reduce comfortably by cutting down their diet, especially that part containing sugar.
INSULIN: THE MATCH THAT LIGHTS OUR FIRES THE story of insulin is a fascinating one, but we can deal with it only very briefly here. Our warmth, energy and muscular activity depend upon the amount of fat and sugar which is burnt inside our bodies. Before fat can be burnt, sugar must be burnt, too—the fat is, as it were, burnt up in the heat generated by the carbohydrate fires. If sugar is not burnt at the same time as fat, the fat is incompletely burnt
and the products of its disintegration are poisonous. They are known as ketone bodies, owing to their chemical composition. This unhappy result takes place either when there is no sugar to burn with the fat, as during a prolonged starvation, or when the sugar, though present in abundance, cannot for some reason be burnt itself.
Insulin is a substance secreted into the blood stream by the pancreas, which makes it possible for the body to burn sugar. Without insulin no sugar can be burnt nor, of course, can the fat be completely used up. The result of this is that sugar accumulates in the blood and overflows into the urine, while poisonous products of fat metabolism, the ketone bodies, appear in the body and may eventually cause death. The cure for this condition which, of course is diabetes, is the injection of insulin under the skin. The insulin puts a match, as it were, to the fires of carbohydrate metabolism in which the fats are completely burnt without leaving a trace of poisonous ash.
If we start at the very beginning of our life, we were each one of us originally one single fertilised cell lying in the ovary of our mother. The ovum or egg cell is like any other cell, a tiny membranous bag containing a mass of jelly or protoplasm. In the centre of the cell is a nucleus which controls the life of the cell. During fertilisation the spermatozoon or male germ pierces the envelope of the egg cell and joins with the nucleus, the two forming one cell. This cell is the fertilised egg and represents our earliest beginnings.
Almost immediately after fertilisation has taken place, the cell starts to divide into two, the nucleus first dividing and the cell then following suit. Each of the two cells so formed is exactly like the one from which it originally arose. In a short time each of these two cells again divides into two, so making four. This process continues, first eight, then sixteen, then thirty-two cells being formed, until the egg comes to look something like a mulberry and is now called a morula.
This process of division goes on continuously until eventually certain parts of the mass begin to take on a definite func
tion of their own. After this, one or two cells become different from the others and ‘differentiate ‘into a rudimentary liver, one or two others differentiate into a lung, and some more start to form a brain.
From this time on these cells are destined to form their respective organs and can only form this kind of tissue. Finally, a complete and perfect individual emerges, an individual composed of thousands of millions of tiny cells, each performing its own special function and working harmoniously with the whole.
This process has been studied to the greatest advantage by means of cinematography. A developing egg is placed under the microscope and supplied with a nourishing medium and a photograph is taken, say once every half-hour. When the photographs are thrown upon a screen, one after the other in rapid succession, the movements and method of division of the cells and the formations into which they arrange themselves can be studied in the minutest detail. The leg of a chicken can be seen to grow from a shapeless mass of protoplasm into a useful member with all its intricate parts, and this even when it has been cut away completely from the main part of the developing egg.
LIFE THAT GOES ON AFTER DEATH
NOT only have quite separate parts of a developing individual been kept alive and studied under the microscope, but organs and separate cells of a fully-grown organism have been kept alive for long periods of time, existing quite separately from their parent mass of body tissue. Thus there is a strain of fibrous tissue which has been kept alive and constantly growing for twenty or thirty years. Some tissues are easier to culture than others, this depending to some extent on the food requirements of the tissue cultures.
All cells, however, are potentially immortal. There is no real reason why they should ever die, provided they are constantly kept supplied with nourishment. Under natural cirumstances the cells of the body die only because they are deprived of their nourishment, for the circulation of the blood and respiration, which are vital processes depending on the activity of the whole body, cease when the individual dies. Life continues for a considerable time in the cells themselves after death of the body as a whole has taken place. If the body of a frog is opened ten minutes after death,
Jthe heart will still be found to be beating. In the same way an animal which is incapable of moving its legs because it is dead can be made to do so some time after death if a nerve passing to the muscles is stimulated by an electric current.
This life after death need not dismay us, for it lasts at most for a few brief hours, and in spite of it, the animal as a whole is dead, for it is capable of no sensation and no movement. Consciousness, of course, is lost immediately, for the brain, being one of the most delicate mechanisms the body possesses, ceases to function instantaneously and dies very rapidly.
Loss of power to function must be understood to be quite different from death. From real death, death of all the cells, there is no recovery, but from loss of function recovery is possible. If the arteries supplying the brain are clamped, consciousness is lost immediately for they cease to function. If the clamps are removed quickly enough, consciousness is regained, for the cells were only temporarily paralysed. If, however, the clamps are left on for six minutes, the brain will never function, for the cells will have died.
Some tissues die more rapidly than others, and it is always the vital tissues that are most differentiated, like the heart and brain, that die most quickly. Others, like the humble fibrous tissue which only supports, may take some hours, and if removed within this period, may live and grow indefinitely if placed in proper surroundings.
We have said that some cells are more differentiated or specialised than others and that these die much more rapidly than their humble, unspecialised brothers. The special cells of the brain and heart and of the liver and kidneys may be compared to the intellectual members of society, such men as scientists, doctors, Cabinet Ministers, and artists, without whose services the community would collapse but whose natures and constitutions are so delicate that they withstand the strains of hardship very poorly. If deprived of their ideal surroundings, they first fail in their work and then in their bodies. The supporting structures of the body, however, such structures as fibrous tissue and fat, may be compared to those humbler but no less important members of society, the manual workers, who being less pampered and less sensitive to their environment are more capable of withstanding ususual hardships.
We have said enough to illustrate how the body is composed of elements which, under ideal circumstances, are capable of
a separate existence, but which must for their proper functioning depend on the integrity of the individual as a whole.
THE PHYSIOLOGICAL INCOME
BEFORE we can understand the part that food plays in the workings of our inner man, we must know something of the chemistry and composition of the food we eat. For practical purposes we may divide our food into four great classes, each of which, as we shall see, has its own particular use. First and most important of all are the proteins. These substances are the essential basis of all living matter, and, as one might expect, their chemical composition is extremely complex. The proteins are found throughout nature in all types of living matter, both plant and animal.
Some of the simpler ones the animal body is capable of elaborating for itself, but others must be obtained by the food, and these will come either from other animals or from plants. If a protein, for example the white of an egg, is digested in a test tube with some juice from the stomach and some hydrochloric acid, after a time the composition of the digest will change. The complicated protein molecule will be split into many simpler parts. If the process is continued until the protein is split into its simplest components, it will be found that, instead of egg albumin, we are left with a large number of substances called amino acids. Here we cannot go into the structure of the amino acids, but we will simply say that they are acids which always contain atoms of carbon, hydrogen and nitrogen, and sometimes also sulphur, phosphorus and other elements. They also have the power of combining together under special circumstances and forming chains.
It must be understood that the proteins are a large class of substances, the individual members of which may have many differences, but they all have certain properties in common. We have seen that a particular protein, egg white, can be broken down into various amino acids. It remains now to reverse the process and synthesise the protein from its constituent parts.
It may be said at the outset that this has never been done completely, but so much of the road has been explored that the remainder is certain. If amino acids of different kinds are made to join together in sufficient numbers, a substance known
as a polypeptide is formed. Many polypeptides joined together in chains form a peptone, and numbers of peptones joined together form primary and secondary proteoses. These latter when joined together make up the complete protein.
Each of these substances, amino acids, polypeptides, peptones, proteoses and proteins have their own special chemical reactions so that they may be identified. The composition of the proteins has been shown by digestion and each stage has been worked out. The road backwards has been fully explored. The road forwards from amino acid to protein has progressed so far only to the polypeptide stage, but the last part may be inferred from the backward journey, which is known. If we remember that there are a large number of amino acids, that they can be combined in any way and any number that nature chooses, and that in a protein molecule there may be hundreds of thousands of amino acids, we shall realise how many proteins Nature can make in her workshops. We shall also realise how extremely complicated their structure is.
SUGAR: A CONSTANT SOURCE OF ENERGY
THE next class of substances are the carbohydrates or sugars. The simplest of these is glucose or grape sugar, and it contains six atoms of carbon, twelve of hydrogen, and six of oxygen. Its chemical formula is C6H1206. Glucose belongs to the class of monosaccharides because it is composed of one sugar molecule. If two monosaccharides are combined together, a disaccharide is formed, of which the ordinary household cane sugar is an example. If many mono- or di- saccharides are combined, a polysaccharide is formed, and the most important of these are starch in the vegetable kingdom and glycogen in the animal kingdom. Starch and glycogen may be resolved by digestion with hydrochloric acid into their component monosaccharides. The carbohydrates are one of the main sources of our energy, for they can be readily burnt up in the body with the resulting evolution of heat or energy.
FATS AND SALTS: THE ECONOMICAL FUELS THE fats are composed of carbon, hydrogen, and oxygen, like the carbohydrates, but they contain a larger proportion of carbon and hydrogen and less oxygen so that when burnt up more heat is generated than when the carbohydrates are burnt. The fats, therefore, are the most economical of fuel. They give us the best money’s worth. As found in nature,
the fats are combinations of a fatty acid and glycerine, a so-called neutral fat. Digestion will result in resolution into the two component parts.
The fourth class of substances are the salts. The simplest example of these is common household salt which, as is well known, is composed of an atom of sodium and an atom of chlorine. Its formula is NaCl. Many other mineral salts enter into our food, but they are not of much interest here, for digestion has no effect upon them and they are absorbed unchanged into the circulation. Their use is a complicated biochemical one and they are certainly essential to life, but we are not concerned with them in this connection.
IF we look for a moment at the single-celled creatures that form the present representatives of the beginning of life, we shall find that a stimulus attractive or repellant applied to any part of the cell will result in the cell reacting as a whole. There is little or no difficulty in communication within the cell from point to point. There is a single-celled creature known as Vorticella which consists of a bell attached by a spiral stalk to some neighbouring weed. A stimulus applied under the microscope to any part of the cell will occasion the stalk to contract and the bell to close up so that the creature takes up a defensive attitude.
When we come to the next grade of living creatures, those that are composed of more than one cell, a difficulty arises.
In the struggle for existence the body must work as a whole, each cell must react as a co-ordinated member if the whole mechanism of the creature is to survive the difficulties of its life. If a noxious chemical substance or another dangerous creature touches one single cell all the other cells in the body must react in an endeavour to flee and escape the danger. For this a service of communications between each body cell and every other cell is required. These communications are known as the nerves and the whole communicating system as the nervous system.
The earliest nervous system consisted in special cells which were situated in the skin of the animal and were connected by a long process running inwards to a muscle cell. Any stimulus applied to the special skin cell would produce contraction of the muscle. This, however, provided a connection between only two cells and each muscle would have to contract independently of the others, so very soon the specialised cells began to branch so that several muscle fibres became connected together. The next step came when the branches became large and numerous and it was necessary to provide a cell both to control them and to nourish them, so nerve cells were formed in the course of the network.
The primitive nervous system thus formed was composed of fibres leading from the special skin cells to nerve cells which were connected together by a branching network of communicating fibres which gave off branches to the muscle fibres. This system was found to be adequate up to a point and is still found to-day in certain lowly animals. But it presented a big disadvantage which had to be overcome before progressive evolution could take place. A stimulus applied to one part of the system might have to travel all round the system and by various devious routes before it could arrive at some point at a distance from where it was applied.
What was needed was a central clearing station or exchange which on receiving impulses could co-ordinate them and pass them on directly to the correct channel. The fibres leading into this central system are known as sensory fibres and those leading out of it to muscles or other organs which produce a reaction are known as motor fibres. The animal thus came to possess sensory organs situated in the skin which sent impulses to an exchange which relayed them by the motor fibres to the muscles or other organs. This central exchange, in order to suit the requirements of the animal,
became formed into a long cord running the whole length of the creature and so a primitive spinal cord was formed.
The path leading from the sensory organ through the nerve cells situated in the central nervous system to the motor organ is known as a reflex are, and the response of a muscle to a stimulus applied to the sensory organ is known as a reflex response. The earliest vertebrates (animals with backbones) and their predecessors, animals like the earthworm, were composed of a number of divisions or segments each of which contained a segment of the central nervous cord which controlled their movements by means of reflex arcs. Thus sensory organs in segment 6 of the animal would send messages to segment 6 of the nervous system and produce a response in the muscles of segment 6 of the animal.
In addition it was essential that each segment of the nervous
system should be in communication with its neighbours and others at a greater distance, so communicating fibres or long tracts were formed which ran the whole length of the central nervous system and co-ordinated the whole. By these means a stimulus applied to segment 6 could now produce a reflex response in one or all of the other segments.
THE EARLY BRAIN: A ‘LOOKOUT’ FOR THE BODY IN the act of locomotion it was found to be of enormous advantage to the animal if that segment which went first, namely the head end, was capable of detecting in advance changes in environment, such as enemies or food. The animal must not wait to discover an enemy until he is already within its jaws. He must know beforehand that an enemy is approaching so that he can make preparations to fight or flee before his enemy is upon him. For this reason that segment of the animal which preceded the rest in locomotion came to be endowed with special sense organs, eyes that could detect variations in light, ears that could pick up vibrations, a nose that could smell and detect food, and organs that could inform the animal of his position in space.
With the development of all these special organs in the head it was necessary that the head should contain a very much larger and more efficient central nervous system than the remaining body segments. This specially enlarged central nervous system of the first segments is known as the brain, and upon its efficiency in warning and producing reactions in the remainder of the body depended the whole survival of the animal in the struggle for existence. The more usefid the brain became the more chance had the animal of surviving. This is well seen if we look at the disaster that overtook the vast cumbersome reptiles that lived in the period following the emergence of animals from the sea. For a time their size and weight enabled them to overcome their adversaries and devour them. Their reaction to environment was to produce enormous muscles in their limbs which would give them power in fighting, and that part of their central nervous system which controlled these muscles—for example, the lower segments of the spinal cord supplying their legs—grew to be enormously larger than the brain itself I
They developed enormous power but their brains failed to develop the skill to use it. Their opponents, on the other hand, left their muscles to look after themselves and developed
their brains, which brought with them new skill and cunning so that the great cumbersome reptiles were wiped off the face of the earth. Man’s salvation in the struggle for existence has been his conservatism in leaving his body to look after itself and concentrating his attention on the development of his brain.
NERVES THAT BRING MESSAGES TO THE SPINAL CORD IN the discussion of the anatomy and physiology of the nervous system that follows, we will start with the parts which are simplest to understand, and lead up gradually to the more complete and more difficult. Situated in the skin are many different kinds of sense organs and still others for receiving sensations of heat and cold. In the muscles are organs that detect deep pressure and tension and in the joints are organs that give information of the position of the bones in relation to one another. From all these sources nerve fibres collect information, one fibre from each organ, and these are later bound together into nerve trunks which travel to the spinal cord and there relay their information.
Just before the sensory nerve fibres enter the spinal cord each fibre gives off a small branch which passes to the cell which looks after the nourishment of the fibre and enables it to live. This cell is an integral part of the nerve and the fibres which reach it from the sense organs and those which enter the spinal cord are really only prolongations of this cell. There is thus a collection of cells which form a slight lump or ganglion on the nerve trunk just before it enters the cord. This is known as the posterior root ganglion, because the sensory roots entering the spinal cord do so in its posterior part (as opposed to the motor root which leaves the cord anteriorly).
Once having entered the spinal cord the sensory nerves do several things. The spinal cord consists of a central mass of grey matter which contains large numbers of nerve cells and a surrounding sheath of white matter that consists of fibres running up and down the cord, some travelling right up to, and down from, the brain. Some of the sensory fibres entering pass immediately into the grey matter and form connections with nerve cells in the front of the grey matter of the same segment. From these, fibres arise which leave by the anterior motor roots and pass into the nerves to innervate the muscles directly.
This is the simplest reflex path. Other sensory fibres run up or down and pass to neighbouring segments, some of them crossing over to the opposite side, forming reflex arcs with other segments. Other sensory fibres, some of which make connections with cells in the posterior grey matter and some of which go direct, turn upwards and run right through the length of the cord to various parts of the brain, carrying impulses of touch, pain, heat and cold, which eventually reach consciousness in the great sensory area of the brain cortex, which we shall talk of later.
LIFE WITHOUT THE BRAIN : THE SPINE’S ACTIVITIES HAVING seen something of the anatomy of the cord we can now say something of what it does, but in order to study this experimentally we must destroy the brain, for this modifies the reflexes produced by the spinal cord by its controlling influence. An animal such as a frog, with its brain destroyed, is known as a spinal animal because its spinal cord only is now controlling it, and as it will live painlessly for a considerable lime we can use it to excellent purpose for studying the functions of the spinal cord.
Such an animal exhibits reflex activity. Thus if the foot is pinched or if a hot instrument is applied to the foot, it will be drawn away, the whole limb being thrown into an attitude of flexion. Thus, the toes and foot are bent upwards, the knee and the hip are flexed. This reflex is known as the flexor reflex and represents a primitive activity of the nervous system in protecting the lower limb from damage. It is the simplest form of reflex, the paths used being those that we have mentioned.
At the same time as the stimulated limb is withdrawn by flexion, the opposite limb will be reflexly extended—the crossed extensor reflex. This enables the animal when standing to preserve its balance on the other foot while it draws the affected one away. Other reflexes may be briefly mentioned such as the scratch reflex in which tickling the back of a dog produces movements of flexion and extension of the hind limb in an endeavour to scratch away the offending object. Also the stepping reflex in which pressure on the sole of the foot, especially if the limb is slightly flexed, produces extension of the stimulated limb and flexion of the other. This is the mechanism of walking. As soon as the right foot touches the ground the pressure reflexly excites
the muscles so that the right foot straightens to bear the weight, while the left flexes in preparation for the next step.
In addition to these reflexes which produce movement, many other reflexes are served by the spinal cord, the movements of the bowel producing peristalsis and the passing of faeces, the contraction of the bladder resulting in the act of passing water, various reflexes resulting in contraction or dilation of the vessels of a part, and a whole multitude of other reflexes which look after our well-being without ever reaching consciousness.
It must be understood that these muscular reflexes, such as the flexor reflex, are shown to their best advantage only when the spinal cord is working independently of the brain, when the brain has been destroyed or when the long tracts connecting it with the cord have been severed. The brain exercises a powerful control over these reflexes, modifying them and preventing them from dominating our existence. In the lower animals they were originally of immense value to the animal which possessed them, but as evolution progressed so have the spinal reflexes become more and more suppressed and brought under control of the higher centres of the brain.
An example of this is well shown in the case of the flexor reflex. In man, if the spinal cord is destroyed or if the nerve fibres running down from the brain which serve the voluntary control of the muscles are destroyed, a flexor reflex may be elicited by scratching the outer side of the sole of the foot. This action causes the great toe to be bent upwards, which, as we have seen, is one of the component parts of the protective flexor reflex. If, however, the sole of the foot is scratched in an individual in whom the tracts to the voluntary muscles are undamaged, the toe will bend downwards and the foot will, as it were, attempt to curl up.
The brain has depressed the flexor reflex and replaced it with a new reflex. It is only when the cord is ‘released ‘from the control of the brain that the flexor reflex manifests itself, although it is there, lying latent all the time. This is an example of what is known as the ‘release phenomenon ‘and is seen right throughout the central nervous system. It is found even in the higher centres of the brain which watch over such recently developed powers as the observance of social conventions, in fact the veneer of civilisation.
There are certain drugs which temporarily paralyse the highest and most recently developed functions of the brain
and release those that lie below. A well-known example of a drug of this kind is alcohol, which is not really a stimulant at all but a paralyser of the highest centres. Under its influence those centres of the brain which supply us with our critical faculties and with the finer conventions of society are temporarily paralysed, allowing our ‘baser ‘and more instinctive natures to appear. It has often been said that alcohol is one of the finest means of revealing the true nature of a man.
THE NERVE CENTRES UPON WHICH LIFE DEPENDS IF the spinal cord is followed upwards to its entry into the skull, it will be found to swell up into a wider mass which is known as the medulla oblongata. This is really only a specialised portion of the spinal cord and works in just the same way. Sensory nerves enter in its posterior part and motor nerves leave in its anterior part. Here, though, are situated the vital centres of the body—the respiratory centre which controls respiration and the cardiac centre which drives the heart. Sensory nerve fibres enter in a nerve, which is known as the vagus, and bring impulses carrying information from the heart and lungs. These fibres make connections with the cells in the medulla, and from them motor fibres leave, those to the heart travelling back to this organ in the vagus nerve, and those to the muscles of respiration passing down into the spinal cord and leaving it at different suitable levels. These nerve centres are absolutely vital for our continued existence, and their destruction brings about instantaneous death.
In this region, too, lie the centres which watch over our balance. Situated in the ear are special sense organs which supply these centres with information of the position in space which our head is occupying and of the movements which the head is making at any moment.
In order that a movement may be carried out in an orderly fashion, it is not sufficient for a given centre only to give orders that a movement must be made. In addition sensory nerves must inform the centre concerned of the progress of the movement from start to finish, so that it may order any modifications that may be necessary. This rule applies, of course, to all movements that are made, the sensory nerves carrying information from the muscles that make them. But, in movement of the head, information comes also from the car, infon jing the centres about the balance of the head and
its position in space. In all the lower vertebrates, these balancing organs are highly efficient and well developed, but in man they have regressed considerably, their powers being taken over by the eyes. They are, however, still of great importance.
Fish have balancing organs of a high degree of efficiency and so have birds, the reason being that these animals live and move freely in space, sometimes freely in three dimensions at one moment. Man, however, has had occasion, up till now, of moving only on flat surfaces. Deprived of the use of his eyes, his balance is precarious, and it is well known that airmen flying in fogs and clouds are often quite unaware of their position in space and may actually fly upside down without knowing it.
THE GREY MATTER: SEAT OF ALL SENSATION ALL the way up the spinal cord, nerves enter from the body and limbs, carrying impulses of touch, pain, temperature and other sensations. Some of these fibres, we have seen, pass to cells in the cord and subserve local reflexes. Others, however, pass up towards the brain and, after relaying in two centres and being joined by similar nerves from the head and face, reach what is known as the sensory cortex of the brain. In their course upwards, they cross to the side of the body opposite to the one where they started, so that fibres reaching the right cortex come originally from the left.
Here we must say a few words about the structure of the brain. It is a hollow mass of nervous tissue, in the centre of which is a cavity containing fluid which circulates downwards to the medulla and then leaves by a small hole and gains the surface of the brain. The cavity is continuous with a central canal lying in the spinal cord. Surrounding the cavity is a great mass of white matter, made up of vast numbers of fibres passing in all directions and connecting together all the centres of the brain. Spread out like a thick plate over this white matter is a,layer of grey matter which contains innumerable nerve cells. This grey matter is known as the cortex and is folded upon itself in convolutions in order to increase its area. The cortex of the brain is the highest and most specialised part of all, and it is this which confers upon us the powers of consciousness.
The sensory fibres, passing into the brain from the spinal cord, spread out and eventually reach a part of the cortex
which runs vertically upwards and downwards just behind the middle of its lateral surface. This area is accurately localised and has been proved by the following experiment to be the site of conscious body sensations. During operations on the brain, this area has been stimulated in a conscious person by an electric current, with the result that sensations of pain have been felt in some part of the body at a distance, say in the hand, if the hand area was stimulated.
The sensory cortex is divided up accurately into areas which receive information from particular parts of the body, and which are the same for all persons. Owing to the fact that the sensory fibres cross over, as we have mentioned, before they reach this area, stimulation of the hand area on one side of the brain will produce a sensation in the hand on the other side of the body.
HOW THE BRAIN GIVES INSTRUCTIONS TO THE BODY JUST in front of the sensory cortex is a vertical strip of grey matter, known as the motor cortex. Here are found large cells of a triangular or pyramid shape, which give off long processes passing right through the brain and spinal cord and making connections with the motor cells in the anterior part of the grey matter of the spinal cord. These are the same cells that we mentioned when we spoke of the motor cells or nerves that the spinal cord contained.
In their course downwards, these long fibres from the motor cortex cross over to the opposite side, so that a cell in the motor cortex of the right side controls a cell in the spinal cord, and through it a muscle fibre, on the left side of the body. This is the reason why a haemorrhage in the motor area of the brain on one side (which is known popularly as a ‘stroke ‘) produces a paralysis of some part of the body on the opposite side. Just like the sensory cortex, the motor cortex has special areas for special parts of the body and stimulation of an area, say the hand area, will produce a movement in the hand of the opposite side.
We have spoken so far only of the two simplest parts of the brain cortex, where some body sensations become conscious and where volitional movement is originated. The rest of the cortex is made up of similar areas which serve their own particular function. Thus the visual area that brings to consciousness what we see with our eyes is situated on each side at the back of the brain. The hearing area is low down on the lateral
surface, and a special speech area is situated on the left side of the brain. The front part of the brain is that part which is used in the processes of intellectual thought, memory and all the higher mental functions of which man is capable. All these areas are connected together by libres which run between all the centres of both sides of the brain, so that a stimulus in affecting only one of them may produce a reaction in any one or all of the others.
THE ESSENCE OF MENTAL ACTIVITY
SO far we have hinted that for any activity of the body to take place, a stimulus must be applied to some sense organ which will initiate a reflex. We have given the impression that, like a slot machine, something must be put in before anything can be got out. That this applies to the spinal reflexes there can be no doubt, but much argument and discussion have taken place over the mechanisms which lie behind conscious and unconscious mental processes. Many of our actions and thoughts appear at any rate to arise spontaneously in the brain without any previous stimulus setting them off, but on further consideration, it will always be found that there is a sequence in our thoughts and actions each depending on one that passed before.
Thoughts that spring into our minds with apparent spontaneity will always be found to have some association, often unconscious, with something previously seen or heard, a combination of circumstances or a previous thought. What constitutes the difficulty in applying our knowledge of reflexes to all grades of mental activity is not this but memory. It is difficult to see how a stimulus applied last year can produce its results to-day. If I agree to meet a friend in a certain place at a certain date and time next year, it is difficult to see how one can ascribe the performance of the visit to ordinary reflex activity. Some say that the reflex has been delayed, others that the nerve cells store up energy which is discharged at the appointed time, but these theories only beg the question by answering it in terms of another.
The best theory on which it is explained is that of the conditioned reflex. If a dog is given food, it will secrete saliva. If a bell is rung, and a few minutes later food is given to the dog, it will again secrete saliva in simple response to the food reflex. But after this process has been repeated a number of times, a curious thing occurs, for it is found that
the dog will secrete saliva even in the absence of food, if the bell is still rung. The secretion of saliva is said to be conditioned by the ringing of the bell, and the reflex in response to which the saliva is secreted is known as a conditioned reflex. The secretion of saliva in response to the ringing of a bell is therefore a conditioned reflex.
It is known for certain that the nerve cells subserving all conditioned reflexes are situated in the brain cortex, for if this is destroyed experimentally in animals, all conditioned reflexes are abolished. It is assumed that new paths are formed between the cells of the cortex concerned and that a given stimulus—light or sound or anything else—therefore travels direct through this new path to the cells which produce the conditioned reflex. It is the laying down of new paths between the untold myriads of cells which constitute the cortex of our brain, in the course of every experience that we undergo, that is the essence of our mental and intellectual activity.
HOW THOUGHTS CARVE NEW PATHWAYS IN THE BRAIN IN the case of memory, a new path is formed down which a stimulus travels at an appointed time, directly to the cells producing the conditioned response. In the case of a prearranged visit, when the arrangement is made a new path is formed between the sensory cells which bring to consciousness the knowledge of time and the motor cells which will set in motion all the activity needed for the visit. The combination of the date and hour when they arrive will occasion impulses which will travel along this specially made path, producing the conditioned response, in this case the visit. It may be argued that we remember such things actually before the time arrives. This is true, but does not invalidate the argument, for some association will have brought the combination of date and time into our mind, producing in an indirect way the necessary conditioned response. Thus, in a chance glance at a calendar a week before the visit was to take place, our eyes might fall upon the date arranged. This stimulus would travel along the path specially laid down and would awake the consciousness of memory. This is only a crude example but serves to illustrate the theory and brings our mental processes into line with the general laws of nature, according to which we know that energy and matter never arise out of nothing.
THE kidneys are the organs that filter off the waste products that collect in our blood, and concentrate them into the urine. We are not concerned here with how they do it; that we must leave to Physiology. Each kidney is a fleshy object that is composed of thousands and thousands of tiny tubes which secrete the urine. This passes from the kidney into a long tube known as the ureter, which leads the urine down into the bladder.
The wall of the ureter, like that of the intestine, is composed of two thicknesses of muscle tissue which by their contraction and relaxation propel the urine down into the bladder. When the ureter enters the bladder it pierces its muscular wall very obliquely so that it has an oblique course inside the bladder wall for about half an inch. While this ingenious arrangement allows urine to flow freely down the ureter into the bladder, it effectively prevents urine from pass-
ing in the opposite direction if large quantities should collect under pressure in the bladder for any reason, thus stopping the development of a back pressure upon the kidneys which might damage them. Sometimes when there is long-continued obstruction to the outflow of urine from the bladder, the bladder becomes stretched so much that this mechanism breaks down and the kidneys are inevitably seriously damaged.
The bladder is the receiver in which urine is stored until a suitable opportunity for voiding it offers itself. The ureters deliver urine from the kidneys at the rate of a drop about every two seconds, so we can readily see that if it were not for the bladder we should spend our whole day passing water continuously. The wall of the bladder is composed of muscular tissue, so that it can contract and relax at will. The circular muscle is collected into a thick ring or sphincter which surrounds the channel which carries urine away from the bladder and is known as the urethra. When the bladder muscle contracts in the act of passing water this sphincter is relaxed so that urine can escape, but at all other times it is tightly closed.
During the time that passes after the bladder is evacuated urine steadily collects inside it from the ureter. The muscle of the bladder allows itself to become progressively relaxed so that the pressure inside the bladder remains more or less at a constant level. Eventually, however, so much urine collects that it is difficult for the bladder to relax any further, so the pressure suddenly rises. The nerves in the bladder then send messages to the spinal cord complaining of the increase in pressure and asking for the bladder to be emptied. These impulses reach consciousness and we can either empty the bladder if we wish to or, if the opportunity does not nrise, we can consciously relax the bladder still further and so reduce the pressure and the demand for evacuation.
This conscious control over the bladder is only learnt by experience and education, and its lack in infants explains why they pass their water at regular intervals. It is only when they become older and acquire this conscious control that they become ‘clean.’ It is never wise, therefore, to scold or punish a child for being ‘dirty,’ because it is not his fault. It is only that this conscious control has not yet developed or has been retarded by some special circumstances.
When water is passed the bladder walls contract and the sphincter closes, and at the same time the muscles of the abdomen contract under the control of the brain so that their
effect is added to that of the bladder. The urethra is simply the tube which carries the urine away from the bladder. In the male it is long and opens at the end of the penis, but in the female it is very short. It is owing to the greater length of the canal in the male and the presence of a gland called the prostate which surrounds it in its upper part that it is so much more frequently diseased in men than in women.
FOOD taken into the mouth is first ground into smaller and smaller fragments by the teeth and intimately mixed with saliva and then finally swallowed. The mouth, of course, is not only concerned with eating but also with speaking and sometimes with breathing when for any reason the nose is blocked. The dangers of habitual mouth-breathing will be readily recognised when it is realised that all the good offices served by the nose that we have previously considered are eliminated by breathing through the mouth.
The saliva is produced by three sets of glands on each side, each of which discharges its secretion into the mouth by means of a duct. One of these is a gland lying on the side of the face in front of the ear and known as the parotid. This is the one which becomes enlarged during the course of the illness known as mumps. The saliva has a double function, for it is not only a digestive juice which helps to split up starch but it also acts as a lubricant and facilitates the movements of the tongue so that speech is more easily accomplished.
When the food has been ground up and mixed with saliva it is rolled up into a ball by the tongue and promptly swallowed. This simple remark has a lot behind it, for in this process many interesting mechanisms are brought into play. After being collected into a ball the food is thrown against the back of the pharynx, where it comes in contact with a very sensitive spot. Immediately a reflex is produced which accomplishes several very important acts. First the soft palate is raised so that the back entry into the nose is closed, preventing food being lost by this route. Secondly, the upper entry of the larynx is closed in rather a complicated way so that no food can enter there. Thirdly, the food is swallowed into the one remaining channel left open, namely, the gullet.
This is a tube with a very muscular wall, one layer of muscle lying longitudinally and another encircling it from top to bottom. During the action of the reflex that we have just mentioned, the muscle above the food contracts and that below it relaxes, so that the food is rapidly pushed downwards. This muscular action proceeds in a wave passing down the whole length of the gullet until the stomach is reached.
It is this mechanism which is responsible for the pro-
pelling force which causes the food to pass from one end of the alimentary tract to the other. It is known as peristalsis and is essentially a wave of contraction preceded by a wave of relaxation into which the food is pushed. All through the alimentary tract from the mouth to the anus where the food or its remains are eventually voided, the same arrangement of muscles is found, and everywhere the mechanism of propelling the food is the same.
THE SECOND MILL THAT GRINDS THE FOOD BETWEEN the gullet and the stomach is a ring of muscle which, when it contracts, firmly closes the upper end of the stomach and prevents the food from being regurgitated back into the gullet. This sphincter, as the muscle is called, is normally kept closed, but during swallowing it forms the lower end of the wave of relaxation which precedes the oncoming food which is thus finally passed into the stomach.
At the lower end of the stomach there is another sphincter which is called the pylorus and which works in exactly the same way as the first that we have just mentioned. There is a slight difference, however, for any solid particles in its neighbourhood have the power of making it contract and close, ensuring that food is not passed on to the next part of the bowel until it is in a more or less fluid condition and in a fit state to be digested by the special juices which are next going to work upon it.
Thus when food has entered the stomach both sphincters close and now begins a firm pummelling in which strong contractions of the stomach pass from left to right over the
surface of the organ, breaking up any solid or semi-solid particles of food and intimately mixing them with quantities of juice which is secreted from the inner surface of the stomach and which helps to make the mass almost liquid in consistency.
The food remains in the stomach for about three hours in normal circumstances, so that it forms between meals a sort of reservoir which is almost constantly passing material into the bowel for digestion. Before breakfast, however, if nothing has been taken all night, it should invariably be empty. When the food has been reduced to a more or less fluid pulp a wave of peristalsis relaxes the pylorus and a quantity of food is passed through the opening. This process is repeated at intervals until all is finished.
THE LAST STAGES OF THE FOOD’S JOURNEY THE tube which next receives the food is called the small intestine owing to its narrow width. Its upper part, next to the stomach, has a special name—the duodenum— which is of interest because quite often a serious ulcer may develop there. Leading into the duodenum are two long ducts, one from the pancreas which secretes a special alkaline juice, and the other from the liver which conveys the bile. Just after the bile duct leaves the liver where the bile is made, it sends off a large duct which passes to the gall bladder.
When the duodendum contains little or no food the bile is side-tracked up this channel and stored in the gall bladder, where it is available in large quantities for the next time it is required when the gall bladder contracts and drives the bile into the duodenum. The gall bladder has the power of absorbing water from the bile which lies in it, so that the bile is concentrated when it is ready for use. Sometimes, especially when there is infection and germs are present in the gall bladder, the bile becomes too concentrated and stones form, leading to a train of serious symptoms.
In the small intestine the food becomes progressively more and more fluid, for not only are the bile and pancreatic juice added to it, but the intestine itself secretes a juice, the three together finally breaking the food down into its last constituent parts, all of which become completely dissolved.
The inner surface of the intestine is raised into many ridges and the whole surface is covered with innumerable tiny finger-like processes known as villi which increase a
thousandfold the surface available for absorbing the disintegrated products of the food. The special function of the villi we shall see later. At the lower end of the small intestine all the valuable parts of the food have been completelyabsorbed. but the contents are still fluid. These now take a new character and are more or less the same, apart from their consistency. as when they are finally voided at the anus. They are now known as fasces.
SOMETHING ABOUT APPENDICITIS
AT the lower end of the small intestine is another sphincter which holds back the remains of the food. At intervals, and especially when food is taken into the stomach through the reflex mechanism, the sphincter opens and the fasces are pushed down into the next part of the bowel, which is known as the large intestine, owing to its greater width. The small intestine enters the large at a right angle and a small portion of the large gut, known as the caecum, projects down below this entrance.
It is from the lower end of the caecum that the notorious appendix springs. This is a small tube about four inches long with a blind end, and it is when this becomes blocked for any reason that inflammation starts inside it and appendicitis, with all its serious consequences, begins. The contents of the large intestine and appendix are always swarming with highly virulent bacteria, even in perfectly normal persons. So long as the germs remain only inside the intestine no harm results, but as soon as they begin to pierce the coats of the intestine and work their way through into the large cavity in which the intestines are confined, the infection spreads all over the abdomen, with the most disastrous consequences.
The inner surface of the bowel wall is quite immune from their advances, and it is only when this becomes damaged by products which have been dammed back for any reason, that any invasion of the walls can take place. We can never tell from outside exactly how far the inflammation has progressed, although we may know that it is inflamed.
AN AILMENT THAT CAUSES UNNECESSARY CONCERN THE remainder of the large intestine has the power of absorbing water and so concentrating the faeces and making them solid so that they can be more easily held until an opportunity arises for voiding them. It also secretes large quan-
titics of slimy mucus which protects the wall from ulceration and lubricates the now solid fasces so that when they are voided this action is accomplished with the minimum of difficulty.
When food is taken into the stomach a reflex wave of peristalsis is set up in the large intestine which pushes some of the fasces downwards and results in a desire to void them. This is the reason why most people always have a desire to pass their motions after breakfast. If the impulse is neglected the intestine becomes accustomed to their presence in its lower part and the desire passes, perhaps not to be repeated for many hours. It is the breaking of this habit of passing one’s motions at regular times that so often leads to constipation.
Constipation of itself leads to very few ill effects and much nonsense has been written of its serious consequences, with the result that an immense amount of unnecessary worry has been produced. This worry about the bowels is of much more importance than the constipation itself. However, it is obviously much more healthy to discharge the fasces at regular and fairly frequent intervals, and for this reason the habit should be carefully preserved.
We have now discussed the whole of the alimentary tract from top to bottom, from mouth to anus, and it may be of some interest to note that the journey has been a long one, for we have passed through nearly forty feet of sinuously coiled tubing.
WE have talked of the mechanism by which air is drawn into the lungs by enlarging the diameters of the chest, and we must now describe the passages which convey the air down to the lungs. During normal breathing air is first drawn into the nose which has a special action in warming and moistening the air and clearing it of some of its larger impurities before it reaches the delicate tissues of the lungs. When the air enters at the nostrils, it finds itself in quite a large cavity whose depth is represented on the front of the face by the distance from the nostrils to the bridge of the nose.
If you can imagine a cavity as deep as this and perhaps one and a half inches from side to side, extending directly backwards until it joins with the cavity of the mouth, you will have quite a good idea of the anatomy of the nose. This cavity is divided into two by a partition which runs from the roof to the floor and completely separates one side from the other. At the entrance of each nostril there are a large number of very stiff hairs which strain off any particles such as soot which might be breathed in.
The side wall of each compartment of the nose is thrown into a number of large folds which greatly increase the surface which is presented to the air passing over it, and as they project inwards towards the partition they take up a lot of room and in places almost touch it. The whole of the interior of the nose, including these folds, is covered with a thick, velvety layer of mucous membrane which is very thickly supplied with blood vessels. It is called mucous membrane because it is a membrane made up of cells that secrete a thick viscid fluid known as mucus. As the mucous membrane is well supplied with blood it is always kept warm and the mucus secreted keeps it soft so that the air passing over it has the chill taken off and is moistened.
In addition some of the dirt carried in with the air adheres to the mucus and is discharged from the nose when next it is blown. That this is so is well shown by the difference in the dirt on our handkerchiefs after a day spent in the smoky atmosphere of a large city and that found after a day spent in the country. It must be realised that all this filth would have entered the lungs, where it might have done
considerable damage, if it had not first been removed by the nose.
The nose is, of course, one of the main places where germs enter the body, and considerable numbers are caught and held up in the mucus which covers its surfaces. Very frequently these are germs which cause trouble and a cold in the head is the result. When this happens the mucous membrane of the nose becomes swollen, so much so that the nose is blocked and breathing through the nose becomes difficult or impossible. At the same time the flow of mucus is greatly increased so that large numbers of the disease-producing germs are washed away in it.
THE SECRET OF A NASAL VOICE
WE have seen that the nose communicates at the back with the cavity of the mouth. The posterior part of the floor of the nose is formed by a muscular sheet known as the soft palate. When this is pulled up, it shuts off the back of the nose from the mouth so that when we swallow fluids they cannot enter the nose and flow out through the nostrils.
Leading out of the cavity of the nose are quite a number of fairly large cavities, whose function it is to make the sounds formed by the larynx reverberate, so that the voice takes on a ringing character. When we speak the nose and mouth are kept in continuity because the soft palate is kept open and air passes through the nose as well as through the mouth. If for any reason the nasal cavity is blocked, as for example when we have a cold in the head or when we pinch the nostrils together with our fingers, the voice will be flat and ‘nasal ‘in quality. The action of these cavities is exactly like that of the large cavity which is always made in the body of a violin behind the strings. One can readily imagine the flatness of the note of a violin that was made of a solid block of wood.
THE NERVES THAT ENABLE US TO SMELL
THE roof of the nasal cavities is formed by quite a thin plate of bone which separates the nose from the cavity in which the brain lies. Piercing this layer of bone are numerous tiny nerves which carry sensations of smell to the brain. Being extremely delicate structures Nature has packed them away in an inaccessible region of the nose where the least possible harm can come to them, and yet where the air warmed
by the remainder of the nose and carrying the odorous substances which stimulate these nerves can easily reach them.
Owing to the close proximity of the brain to the outside wall at this point it is one of the situations in which the brain is most exposed to infection, and it is one among others at which it is supposed the germs of infantile paralysis enter the nervous system. This disease is an example of many such that are caught first in the nose, just like the common cold.
THE SOUND BOX OF THE HUMAN VOICE
BOTH the nose and the mouth lead backwards into a cavity which connects the two and which is known as the pharynx. This has two large tubes leading from its lower portion. The front one is the larynx, which produces the voice, and the one lying behind is the gullet. Leading out of the side wall of the pharynx on each side is a short tube that connects it with the inside of the ear. This is one of the danger spots of the upper respiratory passages, for quite often when these are inflamed owing to some infection, such as measles, this tube also becomes inflamed and infection may reach the cavity of the ear. This is the explanation of the relative frequency of that serious condition known as mastoid disease which so often follows otherwise trivial diseases of this region.
Leading out of the lower part of the pharynx is the cavity known as the larynx. This is like a box with its upper and lower ends open and is formed by a number of small cartilages, the largest of which projects on the front of the neck and is known as the Adam’s apple.
Stretching from front to back of the lower part of this cavity are two folds of mucous membrane known as the vocal cords. When air passes over these structures during expiration and inspiration they are thrown into vibration and a sound is produced. They are well supplied with different sets of muscles so that not only can they be separated and brought closer together so that they may be made to meet completely, but also they can be both stretched and relaxed. Separation and approximation of the cords will control the volume of air passing over them so that the loudness of the voice can be controlled. Tightening and relaxation of the cords determines their pitch so that all kinds of variation in the tone of the voice can be produced. The larynx pro-
duces only the sound of the voice. It is the mouth and tongue, lips and teeth, that fashion these sounds into words and syllables.
WINDPIPE AND BRONCHI: THE SENTINELS OF THE LUNGS THE lower part of the larynx leads directly into the windpipe which runs down through the lower part of the neck into the chest. Here it divides into two branches and these in time divide into even smaller and smaller branches until eventually they reach the air sacs of the lungs.
The windpipe and bronchi are composed of rings of cartilage separated in the intervals by fibrous tissue. The rings are incomplete behind and the interval between the ends of the ring behind is filled in by muscular tissue, thus enabling the tube to be made smaller or larger in diameter when the muscle contracts or when it relaxes. This is of little importance in the windpipe, for it is a very wide tube and only slight variations can be made in its calibre. But in the smaller bronchi it is of some importance, for it is the explanation of how asthma is produced.
There are certain substances in the air we breathe which will cause a spasm of these muscles in individuals who are susceptible to them. Such things are the pollens of certain plants and dusts from various sources. When the substances gain access to the bronchi the bronchial muscles go into spasm in an effort, as it were, to exclude the dangerous particles from the lungs. So effective is the spasm that it often makes breathing almost impossible for the unfortunate sufferer.
It is really misdirected energy on the part of the bronchi, for the condition they produce is more unpleasant and more dangerous than the one they are seeking to avoid. It is therefore justifiable for the doctor to abolish this spasm by the use of drugs, and this is usually done by inhaling the smoke from certain leaves or by giving a special injection. When the bronchi become inflamed the well-known condition of bronchitis is produced.
The mucous membrane of the windpipe and bronchi is very remarkable and forms the final barrier which tends to prevent dangerous substances entering the lungs. It is composed of a layer of tall cells fitted together like a palisade. Each cell has a number of hairs sticking out into the cavity of the windpipe, which the cell can move freely. There are small glands lying deep in the membrane which throw out
quantities of fluid mucus in which foreign particles and germs become entangled.
The hairs on the cells beat always upwards so that they carry the mucus which is laden with foreign particles upwards in a stream away from the lungs. This is eventually voided by coughing, because when it reaches the sensitive larynx a reflex is set up which results in the production of a sharp cough. This same reflex is responsible for the unpleasant results that follow food ‘going the wrong way,’ for instead of being swallowed normally it is inhaled by mistake into the larynx from which it might reach the lungs if it were not immediately coughed up.
THE RHYTHMIC WORKING OF THE LUNGS
THE lungs consist of a vast number of infinitesimally small air sacs crowded together and welded into one mass so that a structure something like a very fine sponge is produced. Each tiny air sac has a correspondingly small branch of a bronchus leading into it, so that during inspiration and expiration the air lying in the air sacs is constantly being changed and kept fresh. The walls of the sacs are formed by a very thin membrane formed by flattened ends which lie only one layer deep, so that an effect like a pavement is produced.
Running in the walls of the sacs and separated from their air-containing cavities by only this thin layer of membrane are innumerable capillaries which are branches of the pulmonary artery that brings impure blood from the right side of the heart for purification. So fine and so numerous are the capillaries that an enormous surface of blood is exposed to the action of the air in the sacs, with the result that the gaseous interchange between blood and air can take place to the best advantage.
The substance of the lungs is made up mostly of elastic tissue, so that when they have expanded under the influence of the increased size of the chest during respiration, they will automatically collapse and drive out the air inside them, when the expanding force is removed. The outer surface of the lungs is covered with a smooth, glistening membrane known as the pleura. The whole of the inner surface of the chest is also lined with this same membrane, the result being that the lungs can move about inside the chest when they are expanding and relaxing with the minimum amount of friction.
BEFORE the inter-relationships and anatomy of the various organs of the body can be fully understood a working knowledge of the system which carries blood and nourishment to every part of the body must be discussed. The vascular system comprises the whole of this apparatus, and in it are included the heart, the arteries which convey pure, nourishing blood from the heart, the capillaries (or fine veins) where the nourishment is given off to the tissues, the veins which convey blood back to the heart, and a specialised system of vessels called the lymphatics. The system is really a closed circle, or rather two circles, as we shall see later, with the heart at the centre driving blood round and round.
If we start with the impure blood returned to the heart by the veins we find that it passes successively through two chambers situated in the right side of the heart and thence by the pulmonary artery to the lungs where it is purified and saturated with oxygen. After leaving the lungs it passes by the pulmonary veins back to the heart, where after traversing two chambers in the left side of the heart it leaves by the great artery called the aorta and finally reaches the tissues by passing through the smaller arteries. After supplying the tissues with oxygen it becomes impure and is carried back by the veins to the heart and lungs for purification.
It will be seen that in the greater part of the body the arteries contain pure blood and the veins impure blood. The arteries may be compared with the clean water supply of a town, while the veins are the sewers. In the lesser circulation—that is the part in which the lungs are interposed— the reverse is the case, for the pulmonary artery contains the impure blood and the pulmonary vein the pure blood. After this general discussion we can now start on a consideration of each separate part.
THE ROUTES BY WHICH BLOOD TRAVELS TO THE HEART IF we start with the capillaries to which blood is brought by the arteries, the microscope must be used, for they are exceedingly small. Every organ in the body is riddled with these tiny tubes which are the means of nourishing the tissues.
Their walls are excessively thin, for they are composed of flat cells which lie only one cell deep so that a kind of pavement is formed, so delicate that nourishment easily passes through it. Further on the capillaries join together, forming larger vessels which in their turn join until eventually quite a large vein is formed. Veins from all parts of the body converge upon the heart, becoming larger and larger as more tributaries enter them. Many of these veins have special names, but only a few need be mentioned here.
Blood coming to the heart from the head is carried by the jugular veins, internal and external. These join on each side with veins from the arm called the subclavian veins, forming the innominate veins, right and left. Later the right and left innominate veins join together to form the vena cava superior or great vein of the upper part of the body, which directly enters the heart.
From the lower part of the body a vein from each leg— the femoral vein—after passing into the abdomen from the thigh and receiving many tributaries, joins with its fellow of the opposite side to form the inferior vena cava or the great vein of the lower part of the body. This vein lies in front of the vertebral column in the posterior part of the abdomen and runs directly upwards until, after receiving impure blood from the liver, it pierces the diaphragm and immediately enters the heart. We have now, by devious routes, brought all our blood back to the heart, so we must next proceed to describe this organ in detail.
WORK IN THE PUMPING-STATION OF THE HEART THE heart is divided into two parts—right and left—by a partition or septum which runs down the middle. Each part is further subdivided into two cavities called auricles and ventricles. The auricles are thin-walled muscular chambers which receive blood from the veins and pass it on to the ventricles. The ventricles are thick-walled and contain very strong muscles which form the main pumping-station of the heart and drive the blood all over the body.
Starting with the right auricle, this cavity has two large openings in its right wall, namely the superior and inferior vena; cava;. In addition there is a small opening—the coronary sinus—which conveys venous blood from the heart itself back to the right auricle. After passing through the auricle the blood enters the right ventricle through the valve called
the tricuspid valve. This structure is interposed between the auricle and the ventricle in order to prevent the blood regurgitating back into the auricle when the ventricle contracts.
The valve is composed of three thin membranous cusps which meet accurately at the centre and form a completely watertight joint when the valve is closed. Each cusp is like an umbrella with the convex surface upwards so that blood can flow freely over it when the auricle contracts, but concave on its ventricular surface so that, under pressure, the ventricular blood catches under it and closes it. The working of the valve is purely automatic and is under no sort of control. It depends solely on the difference in pressure in the two chambers. The sequence of events is as follows :
The auricle is distended with blood entering it from the veins; it then contracts and the pressure rises sufficiently above that in the ventricle to force the valve open. The blood then passes into the ventricle which, in its turn, contracts, and when the pressure inside it rises sufficiently high the valve automatically closes, preventing the blood from flowing back into the auricle.
It might be thought at first sight that the valve would itself be turned inside out under the pressure. Nature has guarded against this by providing thick strands of fibrous and muscular tissue which stretch from the wall of the ventricle to the under surface of the cusps. The umbrella shape of the cusps and their inelastic nature also tend to prevent this movement which would be so disastrous.
Having been filled with blood by the contraction of the right auricle, the right ventricle contracts and drives the blood through another valve into the pulmonary artery which leads to the lungs. The muscular wall of the right ventricle is much thicker than that of the auricle, for it has to pump blood all round the lungs. Its thickness, therefore, is a response to the greater amount of work it must perform. The pulmonary artery divides into ever smaller and smaller vessels until the capillaries lying in the walls of the air sacs of the lungs are reached. Here the blood is re-oxygenated and collected into the pulmonary veins which lead the blood back to the heart by entering the left auricle and pouring the purified blood into its cavity.
The cavities of the left auricle and ventricle show no very marked differences from those on the right side of the heart.
As might be expected, however, the wall of the left ventricle is many times thicker than that of the right, for whereas the right ventricle pumps blood only through the lungs, the left must be sufficiently strong to force it through the whole of the remainder of the body.
THE VESSELS THAT SUPPLY BLOOD TO THE BODY THE left ventricle pumps blood under pressure into the aorta, which is the great main artery that leads blood from the heart and delivers it to the whole of the system. Just from the point where the aorta leaves the heart arise two arteries of moderate size which are two of the most important arteries in the body, for they supply the heart. They are known as the coronary arteries, right and left, because they run round the top of the heart like a crown. Should one of these arteries become suddenly blocked death will ensue immediately. This is the cause of death in those cases where a man suddenly falls down dead in the middle of the street. Blockage of certain arteries in the brain will cause immediate unconsciousness and death at a later date, but there is nothing which will strike a man down and kill so rapidly as blockage of the arteries to the heart. It is indeed a sudden visitation of the Angel of Death.
After giving off branches which supply the arms (the subclavian arteries) and the head and brain (the carotid arteries) the aorta turns downwards and, passing down the back of the chest in front of and to the left of the vertebral column, it enters the abdomen. Here it gives off large branches which pass to supply the liver and intestines, and eventually the main stem divides into two branches which pass one to each leg, and are known as the femoral arteries.
THE PULSE : THE HEART’S SECOND GEAR
PERHAPS the best-known artery of all is the radial, for it is from this artery that the pulse rate is commonly estimated by doctors. It lies on the thumb side of the wrist and anyone can easily feel the pulsations that are transmitted to it from the heart. The value of the pulse can at once be seen when it is realised that it gives an immediate indication of the rate at which the heart itself is beating. Whenever the heart is working under difficulties, when it is itself diseased or when it suffers with the body in general during fever, it goes,
as it were, into second gear and beats more rapidly. At each beat it need not pump out so much blood, as it is working faster. The result is exactly the same as that attained when a car goes uphill in second gear. The work done eventually is the same, but it is done with less strain on the engine. The normal pulse rate of an adult is about 70 to 80 beats per minute. Anything over 90 is usually an indication that the heart is in difficulties and must be rested by keeping the patient in bed.
The larger arteries that we have mentioned so far divide continually until finer and finer branches are reached. These lead eventually into the capillaries where the blood is at last made use of. This continual division into smaller vessels means that when the blood comes to be used the surface of the blood that is available for the interchange of nourishment with the tissues is enormously increased. Some enthusiast has worked out that the surface which the blood presents in the capillaries of a medium-sized muscle, such as the biceps, corresponds to something like ten whole pages of The Times.
THE VESSELS THAT CARRY FOOD TO THE LIVER w
E have now discussed the circulation in general, but we have yet to mention a small but important system of vessels known as the portal circulation. In the section on Physiology we shall see that food, after it has been digested, is absorbed into the capillaries of the intestines. These capillaries, as they do everywhere else, gather themselves into veins which form eventually a large vein known as the portal vein, which enters the liver. This vein conveys to the liver, which is the storehouse and factory of the body, the blood which contains the food.
The portal vein breaks up into capillaries again inside the liver and blood comes in contact everywhere with the liver cells which extract the food and make it up into new products. The blood is then re-collected in a further set of veins which enter the inferior vena cava, and from there it soon reaches the right side of the heart.
The liver, therefore, is an unusual organ, for it has three sets of blood vessels whereas other organs have only two. Arterial blood brings oxygen which enables it to live, and the veins carry off the impure blood. In addition the portal vein brings food from the digestive tract which serves the special functions of the liver.
THE LYMPHATICS: THE FIRST LINE OF DEFENCE BESIDES the blood vessels there are small vessels which carry a watery fluid known as lymph. This serves quite a different purpose from that of the blood. Lymph spaces lie between the cells all over the body and they communicate with tiny vessels which collect the lymph and pass it on eventually into the veins. The lymph vessels open into a large lymphatic channel known as the thoracic duct which lies in the chest and which passes up into the neck and discharges its contents into the subclavian vein where they enter the general blood circulation. Some of the lymph vessels that enter the thoracic duct come from the intestines and they carry the fat that has been absorbed during digestion, so one function of the lymphatics is to convey food into the blood stream.
A further reason for their existence is to protect the body from the invasions of germs or bacteria. Before they enter the subclavian vein, all the lymphatics have to pass through a series of filters known as lymph glands which strain off any foreign bodies or germs which may be present in the lymph and destroy them. Thus, if a few germs gain entrance to the body through a small cut on the foot, the germs will be carried into the lymphatics and held up at the groin where the main lymph glands are situated. If the glands are sufficiently strong they will destroy the bacteria, but if the germs are numerous and virulent they may cause inflammation of the glands which will then become enlarged and inflamed or may-even form an abscess.
WHAT A SWOLLEN LYMPH GLAND SUGGESTS THE lymph glands do their best, and even if the germs are not destroyed or an abscess forms, the glands have localised the infection and prevented it from spreading all over the body where it would be much more dangerous. A swollen lymph gland will always suggest that there :s some sore in the region which the glands drains. Thus, swollen glands in the neck are often the result of inflamed tonsils, swollen glands in the armpit are caused by sores on the arms or breast, and swellings in the groin arise from trouble in the feet or legs. Swellings in the groin can easily be felt under normal circumstances by anyone who searches for them, for they are much larger than the glands in any other part of the body, and are the only ones that are large enough to be felt in the ordinary way.