THE CHEMICAL MESSENGERS OF THE BLOOD

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.

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