THE DIGESTIVE SYSTEM.
The digestive system includes all those or gans that are connected with the function of alimentation, the function, that is, which has to do with the preparation of food to fit it for gaining entrance into the blood, and with the separation of the nourishing from the not nourishing portions of the food. What exactly all this includes will be most readily understood by an outline of the course taken by the food from the moment that it enters the mouth, and of the processes through which it passes till all the nourishment is obtained from it that is necessary.
The food taken into the mouth is bruised and broken down by the teeth, being rolled about in the mouth as well, and mixed with a fluid—the saliva—which is poured out by cer tain glands in the walls of the mouth and their neighbourhood—the salivary glands. Then, made up into a mass and well moistened, it is forced into the back part of the mouth, mainly by the action of the tongue, and carried by mus cular movements, constituting swallowing, into the gullet, down which it passes to the stomach. Arrived in the stomach it is submitted to the action of a fluid—the gastric juice,—whose operation is aided by the heat of the parts and by a slow movement of the stomach walls (peri staltic movement), which causes it to move in more or less of a stream along the walls. After the lapse of some time, from one to three or four hours, the process in the stomach le completed, and the food has become converted into a semi-fluid mixture called chyme. Some of its nourishing elements without delay pass into the blood-vessels which line the walls of the stomach; the rest escapes from the stomach into the canal of the small intestine, where it meets with three other juices—the bile from the liver, the pancreatic juice from the pan creas or sweet-bread, and the intestinal juice poured out from the wall of the intestine it self—which attack the substances which have escaped the action of the gastric juice. Along
the canal of the intestine the chyme is pro pelled, forced onward by gentle circular con tractions of the walls of the canal (peristaltic movements); and all along its course there are being slowly abstracted from it all the nour ishing elements it possesses. From the small intestine the remains of the food pass into the large intestine, along which they proceed much more slowly owing to the form of the large boweL During their slow progress much of the watery material that remains is removed, and finally the waste matters, having obtained some degree of consistency, accumulate in a dilated portion at the end of the large bowel, termed the rectum, till they are expelled by an effort of will.
We have then to consider in detail the alimen tary tract or canal extending from the mouth to the end of the rectum, the juices met with at various intervals, the glands which produce them, and the actions they exert upon the food, and the means by which the digested food is made to give up its nourishing portion to the blood.
The alimentary canal is the anatomical name given to the whole length of the canal or passage along which the food is carried. Its average length in the adult is about thirty feet, or about five or six times the length of the body. The mouth, with the teeth and salivary glands, is situated in the head. The gullet, whose upper wide portion, situated at the back of the mouth, is called the pharynx, is chiefly in the cavity of the chest, lying against the back-bone; and it passes through the muscular partition that sepa rates the chest from the belly—the diaphragm —to open into the stomach. The stomach and remaining portions of the canal are placed in the cavity of the belly, which conceals them, and which is therefore called the abdomen (Latin, abdere, to conceal).
The appearance and position of the different parts of the alimentary /canal will be better understood by referring to the accompanying wood-cut and description (fig. 99).