248 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



rudiment, the vermiform appendix, the lumen of which tends to become 

 obliterated with increasing age. 



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 fig. 

 264. The genus Hyrax is remarkable for a pair of caecal diverticula arising 

 from the colon (fig. 265). In the monotremes the rectum terminates in a 

 cloaca as in the sauropsida, 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. 

 The bile is passed to the intestine by 

 the bile duct (choledochal or hepatic 

 duct), but other products are carried 

 away by the blood (internal secretion). 



Fig. 266. Fig. 267. 



Fig. 266. — Diagram of two types of bile ducts, b, gall bladder; ch, choledochal duct ; 

 h, hepatic ducts; i, intestine. 



Fig. 267. — Liver and pancreas of American ostrich {Rhea), after Gegenbaur. ch, 

 choledochal duct; d, duodenum; dh, bile ducts; /, liver; m, stomach; oe, oesophagus; p, 

 pancreas; pd, pancreatic duct; s, stomach. 



The anlage of the liver is a ventral diverticulum (fig. 268) from 

 the archenteron (p. 221), which grows forward from its point of origin, 

 branches again and again, the ultimate branches forming the glandu- 

 lar part of the organ, the proximal parts of the outgrowth giving rise 

 to the bile duct (occasionally multiple) which empties into the intes- 

 tine. As a result of this method of formation the liver is to be re- 

 garded as a compound tubular gland, the lumens of the tubules form- 

 ing the gall capillaries which eventually empty into the duct. This 

 tubular condition is readily recognized in the ichthyopsida, but it is 

 masked in the amniotes and especially in the mammals, in part by the 



^ 



