THE DIGESTIVE SYSTEM. 



REVISED AND LARGELY REWRITTEN 

 BY DAYID WATERSTON. 



APPARATUS DIGESTORIUS. 



The Digestive System. The physical characters and the chemical composition 

 of much of the food taken into the body are such that it cannot at once be utilised 

 by the organism. Before it can be absorbed and used in nutrition it requires to 

 be acted upon, both chemically and mechanically. The performance of these 

 mechanical and chemical changes is known as digestion. 



The term apparatus digestorius (digestive system) is applied collectively to the 

 organs which are concerned in this process, in the reception of food into the body, 

 and in the excretion of the undigested or unabsorbed residue. 



The simple form of digestive system which is found in many of the lower 

 animals consists of a simple tube, passing through the interior of the body, 

 from an anterior or mouth aperture, to a posterior or anal orifice. The wall 

 and lining membrane of the tube are so constructed as to act mechanically and 

 chemically upon the food in its interior. 



In man, a tube of this kind forms the basis of the digestive system. It 

 extends from the mouth, through the neck, thorax, abdomen, and pelvis, to the anal 

 orifice. But the tube, originally simple, has become modified, in different directions 

 in different parts, for the performance of the various stages of the complex 

 processes of digestion, absorption, and excretion. 



The principal modifications which it has come to present are the following : 



(1) The tube is very greatly elongated, so that its total length measures from 

 seven to eight times the length of the trunk. This is effected by the tube being 

 thrown into folds or coils, especially in that part known as the small intestine. 



(2) Certain portions of the wall of the tube have become modified in structure 

 for the performance of special digestive changes. Thus, in the mouth there are 

 found the teeth and tongue, for mastication or triturition of food and for degluti- 

 tion, or swallowing. Further on in the course of the tube there is a dilated 

 chamber, the stomach, in whose wall special glands, called gastric glands, are 

 present, which produce the gastric juices ; while in the succeeding portion, or small 

 intestine, are found the villi very numerous papillary projections of minute size, 

 whose function is largely that of absorption. 



(3) Certain special accumulations or masses of glandular tissue, producing 

 secretions useful in digestion, are situated altogether outside of the wall of 

 the tube, but communicating with its interior by means of ducts, through which 

 these secretions are conveyed. 



The chief of such masses of glandular tissue are the salivary glands, which are 

 placed in the head and neck, and communicate with the mouth ; and the liver and 

 pancreas, which lie in the abdomen, and are connected with the duodenum. 



These glandular masses, though lying external to the wall of the tube, have been developed 

 is outgrowths from it, and the ducts represent the stalks of connexion. 



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