The Digestive System 155 



The small intestine has heavy muscular walls 

 whose histological structure will be described else- 

 where. It opens abruptly, without any indication 

 of a cascum, into the larg-e intestine or rectum. 



The rectum, r, is of about twice the diameter of the 

 small intestine, though this, of course, varies with 

 the amount of fecal matter it contains ; it is nearly 

 straight and possesses much thinner walls than 

 the small intestine, though this, again, varies 

 with the state of collapse or distention. 



At the posterior end of the rectum is a heavy 

 sphincter valve separating that part of the intestine 

 from the cloaca. 



The cloaca, c, is widest anteriorly where it is 

 about as wide as the rectum ; it gradually diminishes 

 in diameter caudad, and appears flattened later- 

 ally. Its wall has the same general structure as 

 the rectum, as will be described below. The 

 mucous membrane posterior to the openings of the 

 genital ducts is thrown into a more or less com- 

 plete, ring-like transverse fold (Fig. 55 G.). In 

 some species there may be a second, half -ring-like 

 fold in the dorsal wall caudad to the more complete 

 ring. The cloaca is divided by this fold into a 

 larger anterior portion, g, and a shorter posterior 

 portion, h ; in the former the mucous membrane is 

 thrown into a large number of small folds that in 

 places form a network; in the latter the mucous 

 membrane has a hard, thick epithelium, with a 

 smooth surface and only a few longitudinal folds. 



