Water is useful in dissolving the other elements, and reducing them to a state of solution which is fitted for the formation of blood, and of the other fluids of the body. We have already seen in what way these various matters are dissolved and absorbed in the primæ viæ, and how they pass into the circulation; that Proteinaceous and Saccharine matters pass into the capillaries of the Portal vein, and thence onwards through the liver; and that fatty matters are reduced to a kind of emulsion by the intestinal juices, and pass through the lacteals into the thoracic duct, by which they are conducted into the circulation at large.
What then are the chief uses for which these matters are required in the system, and what great functions do they fulfil?
The nitrogenous Aliments are needed particularly to supply the growth and waste of the muscular and nervous tissues, which both contain Nitrogen; as also do all the parts of the body, excepting Fat. This waste is continually going on. It depends upon the fact that, after having lasted a certain time, the particles of all these tissues are gradually displaced, oxidized, and conveyed away out of the blood into the urine and other secretions. In the urine these waste matters are found as Urea, Uric acid, and Kreatine.
The starchy and saccharine parts of the food are destined to pass through a series of changes, which ends also in their being burnt and oxidized, maintaining the animal heat, and forming Carbonic acid. Starch passes first into grape-sugar, by taking into itself two atoms of water, becoming C12H12O12. The Ptyaline of Saliva, Pepsin of the gastric juice, and some similar principle in the Pancreatic fluid, are all capable of causing this first transformation. This sugar is more soluble than Starch. When in the blood, it undergoes a further change, the nature of which is not so clear. It is supposed to be into Lactic acid (C6H5O5, HO7) whose equivalent number is just half that of anhydrous grape-sugar, so that one atom of the latter may become two of the former. This important compound was found by Berzelius, in 1807, to exist constantly in the juice of muscle, as well as in the urine and sweat. (Annuaire, 1848, p. 347.) Liebig at first controverted this, but in 1847 he assented to the statement of Berzelius, which had already been further confirmed by the experiments of M. Pelouze. Many modern chemists, among whom may be mentioned Dr. Bence Jones,[32] consider that Lactic acid, or some compound nearly resembling it, is formed at this step of the process of changes connected with the function of respiration. The acid next combines with free Soda, existing in the blood; and this salt is oxidized into Carbonate of Soda and water, just as a Tartrate or a Citrate might be. (Vide page 127.) This has been ascertained by Magnus and Dumas.
Fatty matters are used in the production and renovation of the adipose tissues; and may also, like the last, be burnt and oxidized to support the animal heat.
As a general rule, the diet of a man in health should contain a due proportion of all four kinds of food; for each one of them is essential, and has its proper function in the system. The albuminous material cannot be dispensed with; and is also the only food which will suffice by itself to sustain life. The mode in which it can adapt itself to perform the office of the other varieties of food was ill understood, until explained by the researches of M. Bernard.
From some experiments detailed in a paper read before the Académie Française in 1848, he concluded that the liver was capable of actually producing sugar and fat out of Proteine compounds. For he found sugar to exist in the substance of the liver when none was to be detected in the blood of the Portal vein which proceeds to it. His results have been mainly confirmed by M. Lehmann. M. Bernard states further, that the action of the liver is in some way essential to the assimilation of saccharine matters; for he has found that when sugar is injected into the veins beyond the liver, it passes out unaltered in the urine.
Thus the process of assimilation, whether of albuminous or of saccharine matters, is not so easy and so simple a thing as might at first be imagined. The study of this process is of great importance; and it appears to afford us a clue to the causation of certain disorders of the blood, of which I shall have to speak hereafter.
Upon the regulation of diet, one of the most important of the duties that devolve upon the medical man, it is not my purpose to make more than a few observations.
All kinds of food are less required by the system in inflammatory and febrile disorders; and should then be administered sparingly, or wholly denied, according to the severity of the case. But in Typhus fever long abstinence would be dangerous; the patient is in peril from extreme weakness and inanition, and, being often totally unconscious of his natural wants, requires to be carefully sustained by constant and small increments of animal and farinaceous food.