muscle, heat destroys it. In man and the mammalia, a muscle becomes incapable of contraction at 45° C. At this temperature vital combustions affect the muscular tissues too profoundly, and destroy their properties in a definite manner: the muscle dies.

Excessive muscular work can raise the system to a temperature at which the body can no longer live. This is one of the reasons why a driven animal dies. If the work becomes excessive, so much heat is produced that radiation from the surface of the body and evaporation of the liquids of the economy no longer suffice to keep the temperature at a level compatible with life. The overheated blood poisons the nervecentres; the animal whose body is surcharged with heat owing to too prolonged exercise, dies in a condition similar to that of a man with sunstroke under a tropical sun.

CHAPTER IV.

COMBUSTION.

General Idea of Combustion-Chemical Sources of HeatAncient and Modern Theories-Part played by Oxygen-Oxidation; Hydration; Decomposition-Complexity of the Chemical Phenomena which Produce Heat-Combustible Materials; Food-Stuffs; Reserve-Materials; Tissue-Materials--Results of Combustion-Products of Dissimilation-Products of Incompleted Oxidation; Uric Acid-Elimination of Products of Dissimilation - Eliminating Organs - Auto-Intoxication Dangers of Human Miasma.

I.

WORK and heat do not exist independently in the animal machine. Now work goes on incessantly in us. During repose, and even during sleep, the internal organs are never inactive. The heart beats and expends considerable force in propelling the blood through the vessels; the chest rises and falls in respiration; the stomach and intestines perform peristaltic movements to pass the food-stuffs onwards.

The work of the human body continues then night and day; it only ceases with death, and at this moment also is extinguished the fire of animal heat.

Life is impossible without heat.

The heat from which the human machine draws the force necessary for its existence is derived from combustion which goes on inside the organism. In chemistry, the term combustion is applied to the combination of two or more bodies with each other, accompanied by the production of light and heat.

It is evidently a considerable extension of the meaning

of the word combustion, and a somewhat figurative use, to apply it to the phenomena which cause the heating of the body during work. The chemical combination going on in us is not accompanied by the production of light. The phenomena which produce vital heat resemble fermentation rather than combustion properly so called. For example, they rather resemble the changes which go on in a damp hay-stack, than the phenomena we observe in a burning fire.

The sources of vital heat are chemical combinations of infinite variety.

It has been long considered that all the combustions in the system are due to the action of oxygen upon the living tissue. At the present time we fully admit the capital importance of oxygen in the chemical combinations which are the source of work; but we recognize that other bodies take a certain share in the vital actions capable of producing heat; hydrogen for instance.

Further, many chemical reactions which produce heat are accomplished by the simple splitting up of a substance into two others which entered into its composition. In other cases the combination is limited to the hydration of a substance which absorbs some molecules of water, or to its dehydration by loss of these molecules.*

The problem of vital combustion has then become very complicated of late; we may say that it is somewhat perplexed, and that it is difficult to give in a few words a clear and concise summary of it. It is a chapter of physiology which is being re-written, and we cannot at this moment formulate our conclusions.

All that we are able to say is, that the unceasing work of the internal organs, which constitutes life, is the transformation of one force, heat. This force is itself derived from the chemical reactions which set free the heat contained in a latent condition in the molecules of which the organs of the body are made up, and in the foods which serve to nourish the organs.

The chemical reactions which set free and render

Lambling. Sources of Heat and Force.

sensible by the thermometer the latent heat-energy, consist in the expenditure of two classes of substances: the alimentary substances introduced into the blood by digestion, and the organic substances which form part of our bodies, and which are removed to give place to new substances obtained from the food.

Certain products of digestion which have only just entered the blood are made use of to undergo the chemical combinations which produce heat, and after their chemical constitution has been changed by this combustion, they are expelled from the body without ever having, in a stable manner, become part of our system. They only pass through the system, being changed in the process.

On the day following too hearty a meal, there may be seen at the bottom of the chamber-pot, a yellowish white, or a brick-red deposit. This is made up of very different chemical substances, certainly, from those which were absorbed the night before, but it results from the transformation of the alimentary substances into new products which have been rejected from the system because they were present in excess, and the organs of the body could make no profitable use of them. Here we have a case in which the food-stuffs have furnished the elements for vital combustions.

In other circumstances, however, the combustions take place at the expense of elements which form an integral part of the body. A man who performs violent muscular work after a two days' fast, cannot furnish the quantity of heat needed by this work, from the products of digestion. But the urine of this man, when the work is over, will deposit a sediment similar in appearance and constitution to that from the urine of the man who has dined too freely. In this case the chemical combinations, which have given rise to the heat, and at the same time to the product deposited by the urine, have not been made at the expense of substances introduced into the system from without, but at the expense of the organism itself, and of the tissues which form it.

Since the system is entirely built up by materials

drawn from the daily food, it is not surprising that there are in it substances analogous in chemical composition to those in the food, and that the elements of the body can supplement, in case of fast, the elements usually supplied by the food.

Hence the chemical sources of heat, and consequently the forces from which work proceeds, may arise either from the food, or from the molecules composing the body.

The animal machine is so constructed as to be able to perform its functions for a long period without external assistance. We see the proof of this every day, in the diseases which keep the patient on a very restricted diet for some weeks. We have recently had a striking demonstration from the experiments to which two eccentric individuals* submitted themselves, one of whom was able to abstain from food for 50 days, and in spite of this to perform bodily exercise.

The body is then able to furnish the elements of the chemical combinations which produce heat and muscular work without the aid of food. But if these elements were furnished at the expense of organs which are the essential parts of the machinery we can understand that the latter would very quickly deteriorate and be worn out. There must then be in the body materials which act as a go-between for the food and the organs. These materials, known as reserves, are substances, the dissimilation of which cannot compromise the regular performance of the organic functions. The reserve

materials are the result of a kind of tribute levied every day on the food, and stored up in various parts of the body as in a savings-bank, on which the system can draw when it has need.

The reserve materials are constituted anatomically for the most part of fat; but fat is not the only tissue of the body used for combustion. There are other substances, for instance a variety of sugar known as inosit, which is found in great abundance in muscular tissue, and the combustion of which is one of the sources of

* Succi and Merlatti.