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.