termed autolysis. The autolytic products of the stored material in the tissues are practically identical with those which arise during the ordinary gastro-intestinal digestion.
2. Muscular Work.—The muscular tissue plays the most important part in general metabolism. Not only is muscle the most abundant tissue present, but it is constantly active and is the great energy-liberating machine of the body. Formerly it was believed on the authority of Liebig that muscular work was done at the expense of the protein material, but it has been conclusively shown that the real source of energy in moderate work is the non-protein material, carbohydrates and fats; of these the former plays the greater part in a man on ordinary diet. If, however, the supply of non-nitrogenous material be insufficient, then the energy has to be supplied by the protein and the output of nitrogen is thus increased. Variations in the amount of creatinin and uric acid (both products of muscle metabolism) excreted have been described. In hard work it is sometimes found that there may be no immediate rise in the nitrogen output on the day of the work, but that an increase is manifest on the second or third day after. While the excretion of nitrogen shows no increase proportionate to the work done, the output of carbon dioxide produced by the combustion of the carbohydrates and of the fats is increased proportionately to the work done.
3. Internal Secretions.—Evidence is accumulating to show that the activities of the various tissues of the body are presided over and controlled not merely by the action of the nervous system but also by chemical substances, the result of the activity of certain organs. To these chemical substances, as already stated, the name of hormones has been given.
The hormone which has been most thoroughly investigated is adrenalin, a perfectly definite chemical compound consisting of a secondary alcohol linked to a benzene ring. It is a product of the central or medullary part of the suprarenal bodies. The medullary part of these organs is developed from the sympathetic part of the nervous system, and adrenalin acts as a stimulant to the terminations of the sympathetic nerves which spring from the thoracoabdominal region. These nerves control the small arteries, and the main action of adrenalin is to cause a powerful contraction of these vessels, and as a result a great rise in the arterial blood pressure. For this purpose it is now largely used in medicine. The constant supply of adrenalin in small quantities seems to play an important part in keeping up the tone of the blood vessels, and when, as a result of disease of the suprarenals, the supply is cut off a serious train of symptoms supervenes.
Allied to adrenalin is a hormone derived from the pituitary body. This also causes a constriction of the small arteries except those of the kidney, which it dilates. An increased flow of urine is produced.
In the thyroid gland a substance, iodothyrin, is constantly being produced, and this appears to exercise a stimulating action on the rate of chemical exchange in the various tissues. Under its administration the waste of both proteins and fats is increased. When the thyroid is removed or destroyed by disease a condition of decreased chemical change and mental sluggishness results, accompanied often by nervous tremors.
A difficulty in explaining these symptoms is caused by the fact that in the thyroid are imbedded four small parathyroids, and it is possible that these produce a special hormone. It has been suggested that this exercises a particular influence upon the nervous system, but further evidence is wanting.
The well-known effects of removal of the ovaries or testes on the development and character of an animal is due to the absence of the special hormone or hormones of these structures. These hormones appear to be produced, in the case of the testes at least, not in the true genital cells, but in the intermediate cells, since it has been found that ligature of the duct, which leads to destruction of the genital cells, does not abolish the development of the sexual characters of the animal.
There is growing evidence that from the ovaries different hormones may be produced in varying amounts which play an important part in regulating the phenomena of sexual life.
The thymus gland is a structure lying in the front of the neck, which is best developed at the time of birth, grows very slowly after birth, and atrophies when the age of puberty is reached. In castrated male animals it continues to grow and persists throughout life. There is some evidence that it may exercise some effect upon the growth of the testes, probably by hormone action.
Pancreas.—Within recent years it has been shown that the internal secretion of this organ plays a very important part in the metabolism of sugar. When the organ is completely extirpated the animal becomes diabetic, i.e. sugar appears in the urine and the animal emaciates. How the internal secretion effects the combustion of the sugar is not yet known. Some workers hold that the action of the pancreatic internal secretion is to control the sugar formation in the various sugar-forming organs, of which the liver is the chief, others that it dominates the utilization of sugar as a source of energy by the muscles.
These are some of the best-known examples of the way in which the products of the activity of one organ modify the functions of other organs. In all probability many more examples of hormone action will be discovered, and it will be found that it plays probably even a more important part than the nervous system in the coordination of function in the animal.
Other factors, besides these already dealt with, play a part in modifying the various metabolic processes, as age, temperature, climate, &c. Very little, however, is definitely known about these various factors.
Water and inorganic salts are quite as essential for the well-being of the body as the energy-yielding proteins, carbohydrates and fats. They, however, probably undergo little or no change in the body; they are excreted pretty much in the same form in which they are ingested. Although they are not subjected to any very great change yet they are of immense importance. No animal tissue can carry on its work in the absence of the various salts. Many experiments have been carried out in which animals have been fed on food as free from salts as possible, and, although the food was much in excess of the energy requirements, yet all these animals died, whereas other animals to which similar food with salts was given throve well. The most important acids are hydrochloric and phosphoric, and the most important bases sodium of potassium. Calcium and magnesium are also of importance, especially where bone formation is taking place. Another element of really vital importance is iron, which is required for the formation of haemoglobin.
VI. Excretion
While we know comparatively little of the intermediate stages in the breakdown of the food constituents, and more particularly of the protein moiety, our knowledge of the final products of the metabolic changes excreted is fairly full. The urine is the main channel of excretion for the nitrogenous waste products. CO2, arising for the most part from the metabolism of carbohydrates and fats, is excreted mainly through the lungs. Water is excreted by the lungs, the kidneys and the skin.
So far no entirely satisfactory explanation has been given of how a fluid like urine, having an acid reaction and containing about one hundred times as much urea and generally more than twice as much sodium chloride as the blood, is formed in the kidneys. The urine is a yellowish fluid which varies greatly in its depth of colour, from pale amber to a deep brown. It has a specific gravity of about 1020, varying with the percentage of solids in solution, and it usually has an acid reaction. It is a fluid of complex character, containing, as already mentioned, practically all the waste nitrogen of the body. Among the principal organic substances present are urea, ammonia, purins (uric acid and the so-called purin bases, xanthin, &c.), creatinin, conjugated sulphates, various aromatic bodies and many other substances in small amount, together with the water and inorganic salts.
The following table from Folin gives a good idea of the average composition of the urine as regards the nitrogen-containing constituents, and its variation according to the nature of the diet when this is free of creatin creatinin and the precursors of the purins:—
| Nitrogen-rich Diet. | Nitrogen-poor Diet. | |
| Total nitrogen . | 14·8-18·2 grms. per day | 4·8- 8·0grms. per day |
| Urea nitrogen. | 86·3-89·4% of total | 62·0-80·4 % of total |
| Ammonia nitrogen | 3·3- 5·1% ,, | 4·2-11·7 % ,, |
| Creatinin nitrogen | 3·2- 4·5% ,, | 5·5-11·1 % ,, |
| Uric acid nitrogen | 0·5- 1·0% ,, | 1·2- 2·4 % ,, |
| Undetermined nitrogen | 2·7- 5·3% ,, | 4.8-14·6 % ,, |
Urea, which forms the chief nitrogenous constituent, amounting on an ordinary diet to about 30 grms. per diem, is for the most part formed in the liver, from ammonia obtained either directly from the blood after absorption from the intestine, or resulting from the denitrification of the amino acids. It may also arise in part from the diamino acids and from uric acid.
Ammonia is present in the form of ammonium salts, and forms about 4% of the total urinary nitrogen. It may exceed this amount under certain conditions, for the most part pathological. The ammonia is utilized by the body to neutralize acids which arise during the various metabolic processes.
Purins (uric acid, xanthin, hypoxanthin, &c.) are all members of a series which have as their common nucleus a body which E. Fischer called purin. The most important member of this series is uric acid. It forms about 2% of the total urinary nitrogen. Recent work has shown that it has two quite definite sources of origin: (1) from ingested food containing the precursors, and (2) from the tissue metabolism. The first is known as the exogenous source, and the second as the endogenous. This acid is chemically known as trioxy-purin, and may be regarded as the union of two urea molecules with a three-carbon chain fatty acid. All the uric acid formed in the body is not excreted as such, part being, as already mentioned, converted into urea. The amount which is converted into urea varies
