The gullet or cesophagus is a long tube pass ing from the pharynx to the stomach. Its mucous coat is loaded with very large glands which secrete a quantity of very viscid mucus.
The stomach itself is a greatly dilated part of the digestive system. It may be said to consist of two parts, even in the human subject ; a more complex arrangement is found in many animals, such as the ruminants. The large dilated portion into which the gullet opens is termed cardiac, and the opening the cardiac or oesophageal opening. The whole is lined with mucous membrane, which, in the empty stom ach, is thrown into projecting folds or rugs, but these folds are effaced when the organ is distended with food. In the membrane are innumerable glands which secrete the digestive juices of the stomach. The gastric juice is acid, due to the presence of hydrochloric acid. The ferments called pepsin and rennin, which is necessary for digestion, is secreted by special peptic or enzyme cells of the glands.
The food now called the chyme passes into the small intestine, a tube about 20 feet long. This tube, besides the muscular and mucous coats, possesses an external coat of loose fibrous tissue covered by a single layer of flat cells. This coat is prolonged into and helps to form the mesentery, a membrane connecting various loops of the bowel with each other and also the intestine with the abdominal walls. This membrane is called the peritoneum. The small intestine is somewhat arbitrarily divided into three portions — the upper (duodenum), the middle (jejunum), and the lower (ileum). The mucous coat contains glands very like the pyloric glands of the stomach, called Lieber n's follicles. They secrete the intestinal juice. In the duodenum one finds in addition highly branched glands called Brunner's. •In both the mucous and sub-mucous coats, and generally involving both layers, are found masses of tissue — lymphoid — similar to that found in a lymphatic gland. Their function is probably connected with the blood and the blood corpuscles. Collections of these solitary glands, forming oblong patches about two inches long, are called Peyer's patches. In ad dition to the follicles of Lieberkiihn and the glands of Brunner, there are the liver and the pancreas, which pour their digestive juices into the small intestine. The bile, which is the
secretion of the liver, is formed continually by that organ, but the amount thus formed is in fluenced by the kind and quantity of food taken. The bile is to be looked upon not only as a digestive juice, but as a drain or excretion, whereby effete and useless matter is removed from the body. The pancreas is very similar in structure to a salivary gland. It secretes pan creatic juice which pours together with the bile into the digestive system, because the common gall duct and the pancreatic duct have a common orifice in the duodenum. The mucous mem brane of the small intestine contains, in addi tion, little projections called villi. These are important organs of absorption. This property they share with the whole digestive system through any part of which, and especially through the walls of the small intestine, digested matter in solution passes into the numerous blood capillaries which form everywhere a dense net work. The villi are peculiar, for each one con tains in addition to blood vessels a small lymph ves sel or lacteal.
Nearly all the fat absorbed by the digestive system is taken up by the little cells of the villi, and passes on into the lacteals and thence into eceptaculum chylP and thoracic ducts into the left subdavian vein.
The unabsorbed food, mixed with the various secre tions we have mentioned, now passes into the large intestine, where both diges tion and absorp tion go on, though to a less extent.
The large intes tine is only five feet in length, but its lumen is much greater than that of the small testine. It mences with cum, a dilated part, into which opens a little blind canal (the form appendix), in some animals a large and important structure. The large intestine cends on the right side (ascending colon), crosses over to the left side (transverse colon) and descends again on the left side (descending colon), and makes a bend (sigmoid flexure), and finally terminates in a somewhat enlarged portion (rectum). The mucous mem brane of the large intestine differs from that of the small intestine in containing no villi or Brun ner's glands. Lieberkuhn's and solitary glands are present, but the aggregation of the latter into Peyer's patches is nowhere to be found.