Page:Collier's New Encyclopedia v. 03.djvu/416

LEFT DIGESTION 362 DIGESTION 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 oesophalageal opening. The whole is lined vdth mucous membrane, which, in the empty stomach, is thrown into projecting folds or rugae, 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 fibi'ous 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 tiasue — lymphoid — similar to that found in a Ijrmphatic gland. Their func- tion is 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 Lieberkuhn and the glands of Brunner, there are two very important glandular structures, 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 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 stinicture 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. The unabsorbed food, mixed with the various secretions we have mentioned, now passes into the large intestine, where both digestion and absorption go on, though to a less extent. When food is taken into the mouth it is at once swallowed, unless it is in a solid form. In this case it is chewed into a convenient size for swallowing, for which purpose it is, in addition, mixed viath the viscid saliva and juices of the mouth. Many animals can hardly be said to masticate; such are the car- nivora (dog, cat, etc.), and they are not provided with grinding teeth. As a result of mastication, the food is gathered in the form of a round moist bolus on the upper surface of the tongue. t is now ready to be swallowed. In the llrst place, it is pushed backward by the tongue and seized by muscles, many of which are attached to the hyoid bone. There is a possibility that during swallowing the food may go the wrong way — i. e., it may pass into the larynx and windpipe. It is prevented from passing into it by the elevation of the larynx which pushes its aperture against and under the back of the tongue, which at the same time is pushed backward. In addition, there is a valve called the epiglottis, which is pushed down over the larynx by the movement just described