AL'IMEN'TARY SYS'TEM (from Lat. all mentum, food). In mammalia, that portion of the digestive apparatus through which the food passes from the time of its entrance until its exit from the body. It is lined by a mucous membrane, which extends from the lips to the being modified in each region. (See Mu cot's MEMBRANE.) The alimentary canal begins at the mouth, and is continued into the space called the pharynx, which communicates with the nostrils above, and the gullet or oesophagus below, and with the month in front. The phar ynx is surrounded by three muscles, the constric tors, which grasp the food, and force it into the next portion of the alimentary canal, the oesoph agus. This is a tube composed of an outer layer of longitudinal muscular fibres, and an inner of circular, which extend down to and spread out upon the stomach. These fibres, by a series of peristaltic contractions, carry the morsel of food along into the stomach. In vomiting, there is a reversal of these actions, which ruminating animals can accomplish at will. The oesophagus passes through all opening in the diaphragm, and joins the stomach, which is a pouch curved with the concavity upward, expanded into a eul de sac on the left side (the cardiac extrem ity), and gradually narrowed to the right or pyloric end. it consists of muscular fibres con tinuous with those of the (esophagus, which become thicker toward the pylorus. Its external surfaces are covered by peritoneum, and its thick soft mucous lining, when the stomach is empty lies in folds. Between the muscular and mucon+ layers is a fibrous layer, in which the blood vessels lie before they pass into the mucous layer (See SromAcu.) At its pyloric or right ex tremity the stomach comilmnicates with the small intestine, which is about 22 feet in length, be coming gradually narrower toward its lower end, and ar ranged in convolu tions, which occupy the middle portion of the abdominal cavity, and are kept in position by the mesentery, which at taches them to the posterior wall of the abdomen.
The small intes tine is subdivided into three parts. The first ten inches from the stomach consti tute the duodenum.
Into it open the duct of the pancreas and the common bile duet. Of the remain
ing portion, the jeju num includes about two-fifths and ileum three-fifths. The dif ferences between these last two con sist ill modifications of their internal structure. The tube consists of three layers and the whole is surrounded by peri toneum. See INTESTINE.
The ileum ends at the right iliac region in the large intestine, which is from five to six feet in length. It begins at the pouch called the blind gut or eul de sae (see C.ECITAI , which has a small, wo•m-like appendage (appendix rermifor 9nis); a double valve guards the opening, of the small into the large intestine. The colon passes upward on the right side to below the liver (ascending colon). then crosses from the right hypochondrium across the upper umbilical to the left hypocliondrium (transverse colon), then descends to the left iliac fossa (descending colon), when it bends like an S (sigmoid flex ure), and then joins the rectum at the left margin of the true pelvis. The colon is distin guished by its pouched or sacculated appearance and the presence of an exterior of three flat bands of longitudinal muscular fibres. The peritoneum covers it only in parts. ( See COLON.) The rectum is not saceulated, hut its muscular coat becomes much thicker; at its lower end the longitudinal muscular fibres stop, but the circular fibres become greatly increased, forming the internal sphincter muscle. (See ANUS.) The rectum is not straight, but takes a curved course.
The alimentary eanal thus consists of a con tinuous passage lined by mucous membrane, which rests on a fibrous and muscular basement. Its length is generally about five or six times the length of the body, or, in other words, about 30 feet. It begins below the base of the skull, and passes through the thorax, abdomen, and pelvis, and consists, in brief, of the mouth, phar ynx, (esophagus, stomach, small intestine and large intestine. The above description refers to the alimentary canal in human anatomy; its parts are variously modified in different animals, as will be found in the articles on its several subdivisions. The process of carrying the digest ed food to the tissues of the body is discussed under CIRCULATION.