CHAP. IX DIGESTIVE ORGANS 431 



From the gullet backwards the enteric canal is contained 

 in the abdominal division of the coelome, to the dorsal wall 

 of which it is suspended by a median mesentery (Figs. 102, 

 and 108, 7nes). The greater part of the canal is developed 

 from the enteron of the embryo, and is consequently lined by 

 endoderm ; the oral cavity is formed from the stomodseum, 

 and the cloaca from the proctodaeum (p. 204). Outside the 

 enteric epithelium are connective tissue and muscular layers, 

 the latter formed of unstriped fibres : it is genera,lly charac- 

 teristic of vertebrates that the voluntary muscles are striped, 

 the involuntary unstriped (compare pp. in and 112). 



The immense liver, divided into two lobes {l.lr, r.lr), is 

 situated below the stomach along the whole length of the 

 abdomen, to the wall of which it is attached by a fold of 

 peritoneum. It discharges its secretion, the bile, into the 

 anterior end of the intestine by a tube, the bile-duct, which 

 gives off a blind offshoot terminating in a gall-bladder 

 (p. 69). 



. The pancreas {pan) lies against the anterior end of the 

 intestine, with which it communicates independently by the 

 pancreatic duct. Opening into the hinder part of the in- 

 testine or rectutn, is a small finger-like rectal gland {ret. gl), 

 the function of which is uncertain. In addition to these 

 there are, as in all Vertebrates, minute tubular gastric glands 

 sunk in the mucous membrane of the stomach (p. 131). 



The spleen {spl) is an irregular, dark-red, gland-like body, 

 of considerable size, attached by peritoneum to the stomach 

 (compare pp. 23 and 98). 



Other so-called "ductless glands"— vi\ac\i are also represented in 

 the frog, and the functions of which are not thoroughly understood— 

 are the thyroid in the throat and the thymus in connection with the 

 dorsal ends of the branchial arches ; there are also suprarenal and 

 interrenal bodies in relation with the kidneys (compare p. 145). 



