OF ABSOEPTION. 21 



it externally in the abdomen, serving to fix it in its place, and 

 to facilitate its movements. 



43. The digestion of the food is effected mainly by the 

 action of different humours, which the food imbibes whilst 

 passing through the alimentary canal. These humours are 

 chiefly the secretions from certain bodies, called glands, situ- 

 ated around the digestive tube, and destined to pour into its 

 cavity various liquids or secretions. The number of these 

 secreting organs varies in different animals, but generally, 

 they are sufficiently numerous. The more important are the 

 salivary and gastric glands, the liver and the pancreas. 



44. Finally, to facilitate the action of the digestive juices 

 on the food, it is useful to divide it mechanically. To effect 

 this, nature employs, as is usual, various means. Sometimes 

 the food is compressed merely by the walls of the digestive 

 tube ; in other animals, as in birds and crabs, the food is 

 crushed to pieces in the stomach ; in others, as in man, the 

 mechanical division of the food is effected by means of the 

 teeth situated in the mouth, at the commencement of the 

 alimentary tube itself. These are the masticatory organs and 

 apparatus. 



45. Thus the digestive tube, extremely simple in some 

 animals, is in others very complex, extending from nearly 

 one extremity of the trunk or torso to the other. Never- 

 theless, its greater part is lodged in the cavity of the abdomen 

 (Fig. 5), which in mammals is separated from the thorax by 

 a muscle called the diaphragm, or midriff. Inferiorly it termi- 

 nates in the pelvis (Fig. 90), the interior of which possesses 

 a sort of muscular floor. Behind, the cavity is shut in by the 

 spinal column, and at the sides by broad muscles extending 

 from the thorax to the pelvis. Internally this cavity is in- 

 vested by the serous membrane called peritoneum, by portions 

 of which (the mesenteries) the bowels are maintained in their 

 place, whilst other portions, extending beyond the margins of 

 the stomach and bowels, and thus floating in the cavity of the 

 abdomen, are called epiploons and epiplooic appendages. 



The various portions of the alimentary tube thus formed 

 and located receive different names. Its first part is called 

 the mouth ; the cavity following it, the pharynx ; next follows 

 the gullet ; then the stomach ; and this is followed by the small 

 intestine, itself subdivided into three portions, the duodenum, 

 jejunum, and ileum. After this follows the large intestine, 

 terminated by the anus. 



