336 



ZOOLOGY 



The stomach is usually well differentiated and may consist 

 of one chamber or of several. In the latter case there is a 

 division of labor among the parts. One portion may be highly 

 muscular and supplied with a hardened internal lining for 

 grinding the food (gizzard of fowls, Fig. 164); in such in- 

 stances another portion is glandular. In the ruminants (ox, 

 deer, etc.), there are four chambers in the stomach (Fig. 163). 

 The gastric glands produce an acid secretion which contains 

 certain enzymes acting chiefly on protein foods. 



Fig. 164. 



Pig. 164. Diagram of the stomach and esophagus of the Fowl, o, esophagus; c, crop; p, pro- 

 ventriculus or glandular stomach; g, gizzard or grinding stomach; i, intestine. 



Questions on the figure. — Compare this figure with that of the stomach of 

 ruminants as to complexity. What are the functions of the various portions? 

 What changes take place in the gizzard of flesh-eating birds if they are forced to 

 live on grain? Why is the crop located outside the cavity inclosed by the ribs? 



The food is retained in the stomach by means of a circular 

 (sphincter) muscle at its posterior end where it narrows into 

 the intestine. This latter portion is the principal digestive and 

 absorptive portion of the tract and varies much in length in 

 the various groups in accordance with the nature of the food 

 used, — the vegetable feeders for the most part possessing the 

 longest intestines. Numerous circular or spiral folds of the 

 mucous membrane occur in the intestine. Special absorptive 

 organs (villi) supplied with blood and lymph vessels may cover 

 these folds. Near the anterior end the ducts of the liver 

 and pancreas open into the intestine. The liver is the largest 



