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 C_{12}H_{12}O_{12}. 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 (C_{6}H_{5}O_{5}, HO_{7}) 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.