i8o The Alligator and Its Allies 



caudad to the stomach; (2) a middle; and (3) a 

 posterior, one half inch cephalad to the rectum 

 or large intestine (Fig. 35). 



As might be expected, the general structure of 

 the wall of the intestine is essentially the same in 

 all three regions, the slight differences noticeable 

 being due mainly to variations in the thickness of 

 the various layers. 



The middle and posterior regions have about the 

 same diameter, while the diameter of the anterior 

 region is considerably greater, due partly to the 

 greater diameter of the lumen but mainly to the 

 greater thickness of the constituent layers, espe- 

 cially the mucosa. The mucosa is also thrown into 

 more numerous and complicated folds in the anterior 

 than in the middle and posterior regions; the 

 complexity of the mucosa seems to diminish as 

 the intestine is followed caudad. In the anterior 

 region the mucosa may form at least one half of the^ : 

 entire thickness of the wall, while in the posterior 

 region it ma}^ form less than one third of the 

 thickness of the intestinal wall. The minute 

 structure of the intestinal epithelium will be de- 

 scribed below. 



The chief peculiarity of the intestinal wall is 

 the apparent total absence of a submucosa 

 (Fig. 47). As will be described later, the mucosal 

 epithelium is laid upon the usual bed of fibrous 

 and lymphatic tissue, the tunica propria (Fig. 

 47, tp). 



