Uric acid is only one of the numerous organic products which result from incomplete oxidation, and which we call waste-products of combustion.

The combustions do not cause the complete disappearance of the tissues on which they feed; they change them as does the flame of a fire the coal and wood it consumes. The burning wood gives rise to products of decomposition, the cinders and soot which are found in a burnt-out fire-place. Similarly the system after work contains the products of combustion-or as they have also been called, products of dissimilation, because they are no longer similar to the organic tissues of which they at one time formed a part.

III.

The products of dissimilation, the history of which is still sufficiently obscure, have one character in common, they are all injurious to life, and are rejected from the body as soon as they are formed, just as the cinders and smoke are removed from the fire.

These waste-products are dangerous to the system, and their presence in the blood becomes incompatible with health when there is any excess of them. There is no danger when there is only a moderate quantity, for then the system can quickly relieve itself of them by means of the organs specially charged with their elimination.

The lungs, the kidneys, the skin, and the intestine have among their functions that of eliminating from the blood whatever harmful or useless substances are present in it, whether they have been formed there or introduced into the blood from without.

These four organs are specially charged with the removal from the system of products which are formed everywhere as a result of combustions. The lungs remove carbonic acid, the kidneys urea, the skin lactic acid, etc. All these are the waste products of vital combus. tions. To these three well-known substances it is necessary to add a great many more, of which we know very little. Fresh researches are daily throwing new light on

the functions of excretion, and show the capital importance of the part they play in the system.

It is no part of the purpose of this book to make a complete study of the products of excretion, but it is indispensable for the exposition of our views on the results of work and of fatigue to insist on one point in their history, on the dangers to which the system is exposed when they are accidentally retained in the blood, or when their elimination is incomplete.

Long before chemical analysis had proved the existence of poisonous principles in the waste-products of dissimilation, many clinical facts had indicated that these principles must exist. We have long known that the slightest hitch in the functions of an excretory organ immediately produces a series of accidents due to the retention in the blood of the waste-products which this organ ought to eliminate.

The function, the suspension of which causes the most grave and immediate danger, is that of respiration. Should the lung become functionless for a few minutes, death occurs from asphyxia, which is a poisoning of the blood by carbonic acid. Carbonic acid is the best known and the most abundant of the products of combustion: it results from the combustion of the carbon contained in all the living tissues. The formation of this gas in the blood is continuous, and the system always contains large quantities of it, but the quantity compatible with life is never exceeded, because the lungs eliminate the surplus as fast as it is formed. If the respiratory function is suspended, the poisonous gas accumulates quickly to an extent incompatible with life.

Carbonic acid is not the only poisonous product eliminated by the respiratory apparatus. The air which is driven from the lung in expiration is saturated with aqueous vapour, and this vapour carries with it a product which has not been isolated, and which is formed in very small quantity, but it reveals itself by its unpleasant qualities and its pestilential smell. This product has been called miasma. If we go in the morning into a dormitory in which many persons have passed the night,

we are struck by an unbearably fetid smell, which resembles no other. It is the smell of the miasma exhaled from the lungs of the persons who have slept there. The air is vitiated by it.

The skin eliminates the sweat, which contains 99 per cent. of water, holding in solution salts, chlorides, acids, such as lactic acid and a special nitrogenous acid called sudoric acid. Urea has also been found in it.

Besides the liquid part of the cutaneous excretion, there is a gaseous part, which is no less important. Various volatile acids and a considerable quantity of carbonic acid are exhaled by the skin. But the products of cutaneous excretion which interest us most, those which best establish the poisonous power of the waste-products of nutrition, are at present little known from the stand-point of chemical analysis, and only manifest their existence by the effect produced on the system when their elimination is prevented. Their poisonous properties are shown by the following experi

ment :

Take a large dog, shave off all its hair, and cover its skin with a coat of impermeable varnish or with collodion, in such a manner that no liquid or gaseous product can be thrown off from the animal's skin. In this manner we imprison in the system of the dog all the products which are usually eliminated by the skin. At the end of eight hours, on an average, the animal dies.

Sokolow, the Russian physiologist, who performed the experiment we have quoted, attributes the death of the varnished animal to poisoning by the principles which are no longer eliminated.

The kidneys eliminate a great quantity of the products of organic decomposition. It would take too long to enumerate them all. The chief are the residues of the combustion of nitrogenous substances: urea, uric acid, and its salts, the urates. But urine, like all excretions, contains also a great number of unknown products. In any case no one will dispute the importance of a rapid removal of the products which are carried away by the urinary secretion.

When the functions of the kidney are abolished by disease altering the structure of that organ, urine has no longer the same chemical composition, and in the end does not carry off the substances which are usually eliminated in it. Their composition is changed and simplified; they contain, so to speak, little but water. Urea and the other waste-products of vital combustions, being no longer eliminated in the ordinary manner, accumulate in the blood, and can be detected there by ehemical analysis. Urinary poisoning, or uræmia, soon follows, and quickly ends in death.

The remarkable experiments of Bouchard have established the poisonous properties of urine, and have shown that the injection of this liquid into the veins of a healthy animal can cause its death in a short time.*

The intestine is one of the eliminating organs which must reject the largest quantity of the waste-products of combustion. But being already mixed with a large quantity of food-residues, and also receiving the secretions of the liver, the pancreas, and many other glands, it is very difficult to discover in this mixture how much is derived from the products of dissimilation. A simple observation shows that the intestine receives its share of the eliminated waste-products of combustion. When there is increased combustion on account of excessive muscular activity, there are always more evacuations, and the stools are more liquid. The intestine seems to have been submitted to the action of certain laxative materials, and these materials, since they are not derived from any change of diet, can only come from the organism itself. The products of dissimilation, increased in quantity by exercise, are eliminated by the intestine, and stimulate its contraction, causing more frequent stools.

The functions of the intestine, like those of the lungs, the kidneys, and the skin, cannot be abolished without grave consequences. When the fæcal matters remain too long in the alimentary canal, owing to an obstructive

* Bouchard, les Auto-intoxications.

lesion, there ensues a series of accidents which may be called stercoral poisoning, and which are due as much to the reabsorption of the products of dissimilation, as to the putrid emanations from the alimentary residue.

The four organs, the excretory functions of which we have shortly studied, are not the only ones charged with the elimination of products of which the body desires to rid itself. All the glands can, at a given time, participate in this function, which might be called the cleansing of the body. The presence of poisonous substances in certain secretions has been discovered by accident. By injecting into the carotid artery of a small animal the saliva of a fasting man, grave consequences have sometimes been produced. This shows that the saliva, like the urine, can assist in the elimination of products of dissassimilation which have been demonstrated to be poisonous.

If we seek to summarise the conclusions drawn from the facts studied in this chapter, we can say that the work of the muscles, like that of the other organs, is accompanied by the production of heat, which results. from chemical actions which we may compare with combustions. The living tissues, at the expense of which these combustions have occurred, have changed in their chemical composition, and become noxious to life, and must be rejected from the body under diverse forms and by special organs.

But the products of combustion are not injurious only to the organism in which they are accidentally retained. If absorbed by other individuals, they can produce in them the same bad effects.

We said a few words about the miasma exhaled by the lungs and the skin. These products are present in almost infinitesimal quantity, but possess most powerfully poisonous properties. If several persons are together in a confined place, the air of the place is quickly infected; but the disagreeable odour is not all; the air is vitiated and dangerous to breathe. Hence the evil results of deficient ventilation.