These foods undergo certain preparations previous to being introduced into the system. In the system the food undergoes farther elaboration, to make it fit to enter into the circulation of the blood, in order to supply suitable material for the master tissues. We will now examine briefly the organs and their secretions that convert food-substances into blood, and, by the blood, into tissue. The solvents and diluents of food in the human animal economy are the saliva of the mouth, the gastric juice of the stomach, the pancreative juice of the pancreas, the bile of the liver, and the juices of the intestines--the succus entericus. The digestive apparatus consists mainly of the alimentary canal together with various glands of which it receives the secretions. The alimentary canal commences at the mouth and terminates at the anus. The average length is about thirty feet, about five or six times the length of the body. The part situated in the head and thorax consists of the organs of mastication, insalivation, and deglutition, and comprises the mouth with the teeth, the salivary glands, and the æsophagus or gullet. The parts contained in the abdomen and pelvis consist of the stomach and the small and large intestines. The glands which are most immediately connected with digestion are very numerous small organs, situated in the mucous membrane of the alimentary canal, and the larger glands, such as the salivary glands, pancreas, and liver, whose ducts open on its inner surface. The mouth is included between the lips and the throat, bounded by the lips, cheeks, tongue, and hard and soft palate. It communicates behind with the pharynx, and through the pharynx with the æsophagus. It is lined throughout with mucous membrane. The mouth contains 32 teeth, 16 in the upper jaw and 16 in the lower jaw. The inferior maxillary bone, or lower jaw, is the only movable bone about the head. The teeth have for their functions biting, grinding, chewing, or triturating any hard food substance that may be introduced into the mouth. The tongue is a muscular organ covered with mucous membrane. By its muscular structure it takes part in the process of mastication and deglutition, and in the articulation of speech, while its mucous membrane, with common and tactile sensibility, is the seat of the sense of taste. The tonsils are two prominent bodies which occupy the recesses formed, one on each side of the fauces, between the anterior and posterior palatine arches and the pillars of the fauces. The saliva, which is poured into the mouth and there mixed with the food during mastication, is secreted by three pairs of glands named from their respective situation parotid, submaxillary, and sublingual.