The Kidneys are two in number, and are placed on the right and left of the lumbar vertebræ in the lowest part of the hypochonders. They are glands of a peculiar and very complicated structure. They separate the urea from the blood, and transmit to the bladder the urinary secretion by two canals called ureters. The upper portion of the kidneys is covered by the supra-renal capsules, the use of which is not known.
This function consists in the decomposition, liquefaction, and absorption of alimentary substances; it prepares the nutriment by separating the assimilable portions which are to be mingled with the blood from those which are not fit to enter into the organism. The aliments undergo in the mouth the first change necessary to their introduction into the digestive canal, which is also no less important in relation to their chemical transformation. They are here mixed with the saliva, which penetrates them thoroughly, softens and dissolves them in part, and thus renders their mastication, taste, and deglutition more easy. The saliva also transforms the amylaceous substances contained in the food first into dextrine and then into glycose or sugar; it reduces a portion of the fatty bodies to an emulsion—that is to say, it separates them into particles held in suspension in the salivary fluid, and begins the decomposition which is completed in the digestive canal.