DIGESTION

the series of changes which food undergoes in the interior of an animal so that it is capable of being absorbed and thus nourishing the body. The mouth is provided with teeth incisors and canines for biting and cutting the food in pieces, and premolars and molars for grinding it down so that it can be more easily swallowed. Six salivary glands, arranged in three sets of pairs—the sublingual under the tongue, the submaxillary under the angle of the lower jaw, and the parotid | in front of the ear—pour their secretion into the mouth. The cavity of the throat, or pharynx, is separated from the mouth by the soft palate above and somewhat in front, the so-called pillars of the fauces on each side, and the tonsils behind the pillars. Like the mouth, the pharynx is lined with smooth mucous membrane. Above, it is in connection with the nasal cavity, which is shut off, during swallow ing, by the soft palate; while below it is continued as the cesophagus or gullet, the trachea or windpipe pa ing in front of the cesophagus and communicating with the pharynx by the larynx, the cavity containing the voice organ. The opening is protected by the epiglottis, a little projection over which the food slips during the act of swallowing. The cesophagus, or gullet, a tube about 10 in. long in the normal adult, connect the pharynx with the stomach. It has strong muscular walls, and is lined with mucous membrane which secretes viscid Benen, and is thrown into longitudinal olds. The stomach is the most dilated part of the digestive system, and is the receptacle for the food after it has been masticated and swallowed. Its shape naturally alters with the degree ofits distension, but when moderately distended it is pear-shaped, curved upon itself. It is situated on the left side, at the upper part of the abdomen, just below the diaphragm, which is between it and the heart and left lung. It has a muscular coat, the fibres of which are arranged longitudinally, circularly, and obliquely, and is lined with mucous membrane thrown into numerous folds and containing a great number of glands which secrete the digestive juices. The sub-mucous ti ue between the muscular coat and the mucous membrane has a great number of little blood-ve els to supply the stomach generally, and especially its numerous glands, with nourishment. Its farther, or pyloric, end leads into the small intestine. The small intestine is divided into three parts—the upper, the duodenum; the middle, the jejunum; and the lower, the ilewm. The mucous membrane which lines the small intestine is thrown into permanent folds in the form of crescents or complete rings going transversely round the interior, diminishing in number and in prominence in the lower part. These are called the valvulae conniventes, and they increase the} absorbing and secreting surface area of the mucous membrane. Over the whole of the interior surface, on the valvule and between them, are little projections called villi, present in enormous numbers, but le in size and number low down in the intestine, their function being to absorb the digested food. In the wall of the intestine there are scattered ma es of lymphoid ti ue, called solitary glands, which, however, are not ' glandular at all, their functions being probably connected with the blood. These solitary glands are here and there aggregated together, forming patches of jumphoid ti ue called Peyer’s patches; solitary glands and Peyer’s patches are found in greater abundance in the lower part of the intestine. The mucous membrane also contains glands in great numbers which secrete the digestive fluid of the intestine. Besides the secretion of these smaller glands there are poured into the upper part of the small intestine the secretion of the liver, or bile, and the secretion of the pancreas, or pancreatic juice. The bile is conveyed from the liver by two ducts, and a greater part of it pa es through another duct, termed the cystic duct, to the gall-bladder, where it is stored. From the gall-bladder it pa es to the duodenum by way of the common bileduct, which is joined just as it enters the intestine by the duct conveying the pancreatic juice from the pancreas. After pa ing through the small intestine, the food, mixed as it is with digestive juices, pa es into the large intestine through a valve termed the ileoczecal valve, which prevents its going back. The large intestine is short compared with the small intestine, but its diameter is greater. The internal surface is smooth, and lined with mucous membrane, with the openings of numerous Lieberkiihn’s glands on its surface, while villi are absent. The large intestine is best considered under several divisions: the ceecum, the saccular dilatation of the intestine beyond the ileo valve, into which opens the narrow blind tube called the vermiform appendix; the ascending colon, going up on the right side as far as the liver; the transverse colon, extending right acro the front of the upper part of the abdominal cavity; the descending colon, going vertically downwards on the left side; the iliac colon, a short portion curving into the pelvis; the pelvic colon, a long loop of intestine in the pelvis; and, lastly, the rectum, which is a dilated part just above the opening of the intestine on the surface. The anus, which is the part actually opening on the surface, is usually kept closed by the strong circular muscles which surround it.