CHAP, x DIGESTIVE ORGANS 447 



From the gullet backwards the enteric canal is contained 

 in the ccelome, to the dorsal wall of which it is suspended 

 by a median incomplete mesentery (Figs, in and 117, 

 mes). 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 generally 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 (Mr, 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 



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 rectum is a small finger-like rectal gland (ret. gl\ 

 the function of which is not known. In addition to these 

 there are, as in all Vertebrates, minute tobul&i 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 " which are also represented in 

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

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

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

 interrenal bodies in the neighbourhood of the kidneys, corresponding to 

 the adrenals of the frog (p. 145). 



