DIGESTION, the change which food undergoes in order to prepare it for the nutrition of the animal frame. It is carried on in the higher animals in the digestive system. In some of the lowest forms of animal life (arncebx)which have no special organs, particles of food are drawn into the body and digested. In higher organisms there is a simple pouch which leads inward from the center of the cluster of tentacles; into this fish and other food are drawn and digested, while the undigested parts are afterward voided through the same aperture by which they entered. In still higher organisms, man himself included, this simple pouch is changed into a complex and greatly elon gated tube, which is provided with one aperture (the mouth) by which food enters, and another aperture (the anus) through which undigested matter leaves the body. The mouth in most animals is provided with hard tissues—teeth, beaks for the subdivision of food before it is swallowed. Vegetable feeders, eating tough grains, roots and fibers, have large molar or grinding teeth, while the carniv ora have these same teeth modified so as to present a cutting edge, with which and their pointed canines meat is torn and cut into pieces, which are then swallowed. Below, the cavity of the mouth passes in to the gullet or oesophagus, and in front of this tube runs the windpipe. Food will pass through the pharynx, or the in terior of the throat, into the gullet; and air, during respiration, passes through the pharynx on into the larynx and wind pipe; a valve called the epiglottis partly closes the aperture of the larynx.
The gullet or oesophagus is a long tube passing 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 cesophalageal opening. The whole is lined with mucous membrane, which, in the empty stomach, is thrown into projecting folds or rugw, but these folds are effaced when the organ is dis tended with food. In the membrane are innumerable glands which secrete the digestive juices of the stomach. The gastric juice is acid, and the chief acid secreted is hydrochloric acid. The sub stance called pepsin which is necessary for digestion, is secreted by the whole of the glands.
The food now called the chyme passes into the small intestine, a tube about 20 feet long. This tube, besides the mus cular and mucous coats, possesses an ex ternal 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 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 kiihn's follicles. They secrete the in testinal juice. In the duodenum one finds in addition highly branded glands called Brunner's. In both the mucous and submucous coats, and generally in volving both layers, are found masses of tissue—lyrephoid—similar to that found in a lymphatic gland. Their func tion 7s probably connected with the blood and the blood corpuscles. Col lections of these solitary glands, form ing oblong patches about two inches long, are called Peyer's patches. In addition to the follicles of Lieberkiihn and the glands of Brunner, there are two very important glandular structures, the liver and the pancreas which pour their digestive juices into tie small intestine. The bile, which is the secretion of the liver, is formed continually by that or gan, 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 channel of excretion, whereby effete and useless matter is removed from the body. The pancreas is very similar in structure to a salivary gland. It se cretes the pancreatic juice which pours with the bile into the digestive system. The mucous membrane, of the small in testine contains, in addition to the struc tures already mentioned, little projections called villi. These are important absorb ents. This property they share with the whole of the digestive system through any part of which, and especially through the walls of the stomach and small in testine, digested matter passes into the numerous blood-capillaries which form everywhere a dense network. The villi are peculiar, for each one contains in addition to blood-vessels a small lymph vessel 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 to the blood.