DIGESTIVE TRACT. 



35 



developed, as in grain-eating birds, the muscles of this gizzard 

 develop a tendinous disk on either side, while the inner surface 

 is frequently lined with a firm horny coat which aids greatly in 

 grinding the food. 



In the mammals the line of division between stomach and 

 oesophagus is more sharply drawn than 

 elsewhere in the vertebrates. In the seals 

 alone is the stomach parallel to the body 

 axis ; elsewhere it is twisted into a trans- 

 verse position. In the mammals it also 

 shows greater variations of form than in 

 any other group, modifications doubtless 

 to be explained by differences in food. It 

 may be either a simple sac, or it may be 

 partially subdivided into chambers. In the 

 simpler forms we may distinguish regions 

 in the stomach, the cardiac and pyloric 

 already mentioned, and between them a 

 fundus region characterized by difference 

 in the glands lining the walls. When the 

 subdivision occurs, the chambers corre- 

 spond more or less closely to these glan- 

 dular regions. This division reaches its 

 extreme in the ruminants, where usually 

 four divisions are recognized. These are 

 in order (Fig. 38), (i) rumen (paunch), 

 (2) reticulum (honeycomb), (3) omasum 

 or psalterium (manyplies), and (4) abom- 

 asum (rennet). In the cetacea there are 

 a number of diverticula about the pyloric 

 region. It must, however, be kept in 

 mind that the rumen and reticulum are 

 not truly gastric but oesophageal in nature, and that they serve 

 not as digestive organs, but for the storage of food. 



In the lower forms the liver duct opens close behind the pylo- 

 rus, but in the higher vertebrates a tract of some length, the 

 duodenum, may intervene between the two. While usually con- 

 sidered as a part of the intestinal region, this duodenum is really 



Fig. 37. Digestive 

 tract of a bird of prey. 

 c, crop ; 7, intestine ; 

 m, muscular stomach; 

 pf glandular stomach; /, 

 trachea; v, vent. 



