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. [Illustration: 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. The muscularis mucosa is somewhat variable in thickness and is thrown into folds that correspond to the larger folds of the epithelium and the submucosa; 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 longitudinal 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.