THE STOMACH 



241 



secretion which hardens into a horny (keratoid) lining, sometimes 

 developing into tubercular structures, of great use in grinding the 

 food, thus in part making good the absence of teeth. In the grain- 

 eating birds small pebbles are taken into the gizzard and are used in 

 triturating the food. (In the fossil pterodactyls small clusters of 

 stones are sometimes found in such a position as to lead to the sup- 

 position that these reptiles also had a gizzard.) The gizzard is best 

 developed in the grain-eating birds and is weakest in the birds of 



oe 



Fig. 254. — Outlines of the stomachs of several mammals (various authors), after 

 Oppel, to show the distribution of the different glandular regions. Horizontal lines, 

 oesophageal; oblique, cardiac; dots, fundus; crosses, pyloric; A, Ornithorhynchus; B, 

 gray rat; C, tapir; D, seal; E, whale {Lagenorhynchus); F, mouse; G, dog; H, kangaroo 

 (Macro pus). 



prey. In one species of pigeon part of the wall of the gizzard is 

 ossified. 



The mammalian stomach shows the greatest range of form (figs. 

 253, 254) and the greatest development of different kind of glands. 

 It may be a simple sac or it may be subdivided into a series of cham- 

 bers. In some cases {Ornithorhynchus, fig. 254, ^4) it is almost or 

 entirely oesophageal in character. Occasionally the cardiac glands 

 are absent. It may be a simple sac, longitudinal or transverse in 

 16 



