DIGESTIVE ORGANS. 



231 



there is a relation between the development of caecum and stomach. The caecum 

 becomes enormous in certain rodents and marsupials (sometimes longer than the 

 body) and plays an important part in digestion, being sometimes lobulated or 

 furnished with internal folds, those of the rabbits being arranged in a spiral manner. 

 In man and the anthropoids and some other forms, as is well known, the distal part 

 of the caecum degenerates to a rudiment, the vermiform appendix, which tends to 

 become obliterated with increasing age. 



FIG. 236. FIG. 237. 



FIG. 236. Diagrammatic longitudinal section of the cloacal region of a duck embryo 

 at the twenty-second day of incubation, after Polndyer. ag, anal groove; c, cloaca; cp, 

 cloacal plate; /, bursa Fabricii; p, phallus, with caecal duct; sp, stercoral pouch of rectum. 



FIG. 237. Semidiagrammatic course of intestine of new-born deer Cervus canadensis, 

 after Weber, c, caecum; d, duodenum; co, colon; j, jejunum; m, mesentery. 



Both small intestine and colon are at first straight, but with growth they become 

 longer, involving convolutions varying in pattern and extent in different groups, 

 the patterns of the colon being of some systematic value. The full history has 

 been worked out only for man, two stages being represented in figure 238. The 

 genus Hyrax is remarkable for a pair of caecal diverticula arising from the colon 

 (fig. 239). In the monotremes the rectum terminates in a cloaca as in the saurop- 

 sida, and the same condition occurs in the young of all higher mammalia. In the 

 later stages, however, the urogenital and digestive openings become separated by 

 the formation of a perineal fold between the two. 



THE LIVER (HEPAR). 



The liver, the largest gland in the body, has several functions. It 

 secretes the bile (gall) and forms several internal products such as 

 glycogen, urea and uric acid, of great importance in the animal economy. 



