180 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 may 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). 
