The interior of the small intestine is dotted all over by a huge number of hair-like processes termed villi, each containing a lacteal and a very tortuous capillary blood-vessel. The mixture of soluble carbohydrates, proteins, and fats with the disassociated mineral salts and water, freely bathes these villi and permits of the absorption of the fatty acids, now converted into soaps by union with the alkaline salts of the bile through the lacteals, and the grape sugar, amino-acids, water, etc. through the capillaries. In this way the food reaches the circulation and is conveyed by the blood to the furthermost recesses of the tissues - each cell of which has immediate access to the nutrient fluid.

Just what takes place in the circulation is not quite clear, although certain facts are well known, e.g. that all the carbohydrates pass into the liver, where they are converted into a substance called animal starch or glycogen, constituting a storehouse for the subsequent necessities of the body, chiefly as food for the muscles. Before passing to the muscles it is again transformed into grape sugar where it is utilised, and if for any reason the muscles are unable to make use of it, it is excreted by the kidney and the disease termed diabetes is present.

It is also known that the tissue cells themselves contain ferments analogous to ptyalin, pepsin, lipase, etc., which attack the tissues in appropriate conditions, e.g. after death during putrefaction, but are unable to digest living tissues because of the presence of anti-ferments which counteract their effect.

The intricate changes occurring in the tissue cells themselves, however, are quite unknown. All we know is surmised from the nature of the excretions which are eliminated from the body, the fats and carbohydrates being finally resolved into carbonic acid and water and the proteins into the same substances with the addition of urea, uric acid, creatinin, and sulphates. These end-products are compatible with the belief, which now amounts to a certainty, that the tissue cells themselves are constructed from the alimentary principles and that they are constantly being consumed by a process of slow combustion. This is termed oxidation, because in large measure it takes place under the influence of the oxygen which has been extracted from the atmospheric air and carried in the red-blood corpuscles up to the very walls of the cells. The results of this slow fire are the products mentioned above, which are expelled from the cell into the blood and carried outside the body through various portals - the kidneys, the lungs, the skin, and the bowels.

The cells are thus able to repair their waste substance by appropriating the food supplies in the nearest circulating channel and building them up into their own tissue. It was at one time thought that this could only be effected by proteins, which were the essential food of the cells. But this theory was shattered by two eminent German scientists, Fick and Wislicenus, who in 1865 climbed the Faulhorn, using only non-nitrogenous food during the feat, and finding that their excretion of nitrogen, the peculiar possession of the proteins, was quite insufficient to have produced anything like a tithe of the energy represented by the work done. It is still known that it is impossible to manufacture tissue or flesh substance without protein, but this is wasted in much smaller quantities than was at one time believed. and its expenditure can be very much reduced by a proper reliance on fats and carbohydrates.

It is now well known that the nitrogen of the major portion of protein food is expelled unutilised from the body, the protein having been broken up to obtain, for energy-producing purposes, its carbonaceous molecule. It is hardly fair to allege, and indeed there is no proof to support the hypothesis, that this non-nitrogenous portion of the protein is any more valuable than fats and carbohydrates, and if it be not, then there is no rational basis for eating more than a very limited quantity of protein. It is also a remarkable fact that, except during convalescence from serious illness or for a day or two at the beginning of a holiday, it is impossible to store up as a reserve in our tissues any of the excess protein food we consume, whilst fats and carbohydrates are freely stored up, usually in the form of adipose tissue.

I have already mentioned that exercise does not increase the output of nitrogen, showing that neither protein food nor protein tissue is used for the production of energy. Carbonic acid, on the other hand, is excreted in greatly increased quantities during exertion, and this proves that energy is derived from the combustion of fats and carbohydrates.