MAMMALS. 



367 



A few other facts concerning the teeth may be added. 

 Occasionally, as in rodents and elephants, the incisors may- 

 have persistent pulps, and hence may continue to grow through- 

 out life. The enamel is lacking in a few forms, like the eden- 

 tates and the dugongs. The milk dentition is lacking in some 

 rodents ; in the guinea-pig the milk dentition is shed in utero, 

 and in the seal it never cuts the gums. Finally, there is such 

 correlation between the teeth and other structural features, that 

 the dentition affords an index to the classification, and hence 

 becomes of great assistance to the paleontologist. 



Fig. 351. Diagrams of stomachs of, .-/, horse; B, pig; C, Lagenorhynchus ; 

 D, ziphioid whale; E, seal; F, rat. d, duodenum; 0, oesophagus; p, pylorus; 

 oesophageal region horizontally lined ; cardiac gland region obliquely lined ; fundus 

 gland region dotted ; pyloric gland region with crosses; after Oppel. 



The oesophagus is greatly elongated, extending from the 

 pharynx through the diaphragm to the stomach. Usually the 

 stomach is regarded as the saccular enlargement of the alimen- 

 tary canal, lying between the oesophagus and the intestine ; but 

 when histological and physiological features are taken into 

 account, it is seen that frequently the lower end of the oesoph- 

 agus expands, and takes part in the formation of the gastric 

 enlargement, and that the stomach proper begins only where 

 the gastric glands appear. Of these glands three kinds are 

 recognized, cardiac, fundus, and pyloric, for the characters of 



