These substances go especially to form tissue, and so the nitrogenous substances have been called tissue-forming foods. But they do not solely contribute to that purpose, some part being oxidised in the blood, with the waste nitrogenous substances from the tissues, which are also oxidised in the blood, forming the nitrogenous waste that is got rid of by the kidneys.

No dietary, therefore, is complete without one or more of these nitrogenous foods. Some time ago it was thought that the force we exerted was due to the oxidation of the nitrogenous substances of which our muscle is composed, but it has been shown by experiments made by different observers that this is not the case, but that the force we exert is produced by the oxidation of the non-nitrogenous substances in our foods, of the substances containing carbon, hydrogen, and oxygen, but no nitrogen, and that the greatest exertion can be supported for a time upon non-nitrogenous foods. It is clear that if force were produced by the oxidation of the nitrogenous substances, the amount of nitrogenous waste that we excrete from the kidneys during exercise ought to be greater, in proportion to the exertion. This is not found to be the case; on the contrary, it is the amount of carbonic acid that we excrete, and the amount of water that passes off, that are in proportion to the amount of work done. It is therefore certain that the main object of the non-nitrogenous foods is to produce beat by being burnt in the blood, and thus keep us at a proper temperature, and supply force for the work we do; and the chief use of the nitrogenous foods is to repair the waste of the nitrogenous tissues, and only to aid in a secondary way the production of animal heat; in fact, the muscular tissue of the body is really the apparatus by which force is exerted, and that force is not due to the heat produced by the oxidation of the muscular tissue.

These, then, are the chief substances contained in foods.

The diet of an adult doing an average amount of work should consist of a mixture of the food substances I have just described, in the following proportions: - Substances containing nitrogen, 4 1/2 ozs.; fats, 3 ozs.; carbo-hydrates (starches, gums, sugars), 14 1/4 ozs.; salts, 1 oz. - total, 22 3/4 ozs. of dry substances (containing no water). This, in the form of moist food, is equivalent to about 40 ozs. of food; and, besides this, he should have from 70 to 80 ozs. of water.

These are the quantities necessary when an ordinary amount of exertion has to be undergone; of course, if greater exertion is required, much larger quantities of these substances must be taken, and especially of the carbo-hydrates, and also larger quantities of the nitrogenous substances, though not in proportion to the amount of work performed.

It was found during the construction of the North of France Railway, that the English workmen did much more work than the Frenchmen; and they tried to find the cause, and discovered that the English took a considerably larger quantity of meat with their food; the Frenchmen were then put on the same diet, the result being that they were able to equal their English comrades in the amount of work performed; so it is quite clear that hard work cannot be done without an increase of nitrogenous as well as of non-nitrogenous foods; and this is because the tissues are being used faster, and require to be repaired.

If less than this amount of food be taken, the same amount of work cannot be performed for any length of time. If the amount be considerably less, say 16 ozs. of dry substances, then starvation begins, the tissues of the body begin to waste rapidly, faster than they can be replaced. If more than this quantity be taken with an insufficient amount of exercise the result is what is called plethora, in one form or another. If a great excess of non-nitrogenous food be taken, and little exercise, the result is the production of fat. On the other hand, if too large an amount of nitrogenous food be consumed, with but little exertion, then the consequence is gout or some kindred disease.

I must now speak to you more particularly about gelatine. This has for a great number of years been considered a very important food. It is certainly an extremely digestible substance. Jellies are given to invalids because they can digest them. But a series of experiments was made some time ago in France which threw great doubt upon the nutritive power of gelatine, and which made people begin to think that jellies were of no use at all as food. This, however, is against the experience of ages. It was found, for instance, that dogs would not live upon gelatine, and that they did as well without it as with it. It is not likely that dogs would live upon gelatine; no animal will live upon any one thing. A dog will live upon bones, but not upon gelatine prepared from the bones. More recent experiments have shown that gelatine is an important food in this way, that it can take the place of part of the nitrogenous substances which are being oxidised in the blood. It seems likely that gelatine cannot take part informing tissue in the body, but it is certain that it is oxidised in the blood. So in the case of an invalid suffering from an acute disease, who cannot digest strong nitrogenous food, waste is going on from his body, and it must come from his tissues; he cannot digest ordinary meals, but he can digest gelatine, and this is oxidised in his blood, and prevents the tissues from being wasted so fast. Although it does not go to form tissues it is certainly oxidised in the blood, and so is an important food in cases where other nitrogenous foods cannot be digested.