The alligator and its allies

by A. M. Reese

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, especially 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 described 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).

At the outer border of the tunica propria, and with no tissue corresponding to a submucosa between it and the circular muscular layer, is a thin and indistinct layer that has the appearance of a longitudinal layer of muscle fibers; this should correspond to the muscularis mucosa ([Figs. 47], [48], [49], and [51], mm).

Fig. 47. A transsection of the wall of the anterior region of the small intestine of the hibernating animal, under low magnification; ln, lymph node; tp, tunica propria; other letters as in [Figure 42].

The circular, cm, and longitudinal, lm, muscle layers are compact, and are distinct from the other layers of the wall; the former is approximately twice the thickness of the latter. The relative thickness of all the layers in the three regions of the intestine may be seen by comparing [Figures 47], [48], and [49].

Fig. 48. An outline of a transsection of the wall of the middle region of the small intestine of the hibernating animal, under low magnification; lettering as in [Figure 42].

The serosa, s, which is of about the same character in the three regions under discussion, is a distinct and fairly dense layer of connective tissue with numerous blood-vessels.