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 
