Outside of the mucosa is a thick circular layer of involuntary muscle fibers, cm, the fibers being collected into irregular bundles, between which are narrow spaces filled with connective tissue that contains a few small blood-vessels.
Surrounding the circular layer is a thinner and less clearly defined layer of longitudinal muscle fibers, lm. The muscle bundles are more definite than in the circular layer and are separated from each other by a considerable amount of connective tissue with a few small blood-vessels.
Fig. 41. A transsection through the anterior region of the œsophagus of the hibernating animal under low magnification; bv, blood-vessels; mb, muscle bundles; other letters as in [Figure 42].
The serosa, s, is here quite indistinct. It consists of a slightly vascular connective tissue which cannot be distinctly differentiated from the connective tissue of the longitudinal layer.
Fig. 42. A transsection through the posterior region of the œsophagus of the hibernating animal, under low magnification; e, epithelium; cm, circular muscles; lm, longitudinal muscles; mm, muscularis mucosa; sm, submucosa; s, serosa.
In the posterior region of the œsophagus, as may be seen by comparison of figures 41 and 42, the wall as a whole is about one third thicker than in the anterior region just described, though how much of this difference is due to different degrees of distension or contraction it is hard to say.
The epithelium, e, is in the tissue studied thrown into less complicated folds than in the anterior region, and is not so thick.
The submucosa, sm, if the entire layer may be so called, has about the same thickness and structure as in the more anterior region; but instead of the small and widely scattered bundles of longitudinal muscle fibers there is a distinct layer of muscle which may be called the muscularis mucosa, mm, lying about midway between the epithelium and the circular muscle layer.