The Digestive System 171 



which may be called the muscularis mucosa, mm, 

 lying about midway between the epithelium and 

 the circular muscle layer. 



The muscularis mucosa is somewhat variable in 

 thickness and is thrown into folds that correspond 

 to the larger folds of the epithelium and the sub- 

 mucosa; one of these folds is shown in Figure 42. 

 The fibers of the muscularis mucosa are apparently 

 all longitudinal in position. 



Outside of the submucosa is a layer of circular 

 muscle fibers, cm; it is here somewhat wider and 

 more dense than in the anterior region. 



The longitudinal muscle layer (Fig. 42, lm) is 

 much wider and more compact than in the anterior 

 region. The fibers are indistinctly divided into 

 large irregular masses as shown in the figure. 



The serosa (Fig. 42, s) is a varying but fairly 

 thick layer that is quite distinct from the longi- 

 tudinal muscle layer. It consists of the usual 

 connective tissue groundwork with scattered blood- 

 vessels. 



The epithelium, as was said above, is thicker 

 and somewhat more folded in the anterior than in 

 the posterior region, and in the former region is 

 partially ciliated while in the latter cilia are entirely 

 wanting. With these exceptions the epithelium 

 is practically the same in the two regions. 



Figure 43 represents the epithelium from the an- 

 terior region as seen under high magnification. The 

 outlines of all the cells could not be determined 



