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 caecum, into the large 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. 



