OF ABSORPTION. 21 



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. 4), which in mammals is separated from the thorax by 

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

 nates in the pelvis (Fig. 77), 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, 



46. Acts of the Digestive Function. The phenomena 

 which take place in the different portions of the digestive tube 

 constitute a series of acts or functions all tending to one end. 

 They may be thus classed: 1. There is the prehension of 

 the food ; 2. The mastication ; 3. The insalivation ; 4. The 

 deglutition ; 5. The chymification, or stomachal digestion ; 

 6. The chylification, or intestinal digestion ; 7. Defaecation ; 

 8. The absorption of the chyle. 



Let us now examine these organs and their acts suc- 

 cessively, in man and in the animals which most approach 

 him. 



