THE (ESOPHAGUS. 125 



mucous membrane like that of the mouth and pharynx, of 

 whose walls it forms a portion. The epithelial covering con- 

 sists of squarnous epithelium, except at the upper part of the 

 posterior surface, where it is ciliated. Mucous glands and 

 lymphoid tissue are abundant. 



The (Esophagus. 



The oesophagus is a tube whose walls consist of four coats, 

 mucous, submucous, muscular, and fibrous, from within out- 

 ward. 



The mucous membrane, which when the oesophagus is col- 

 lapsed is thrown into longitudinal folds, consists of the epithe- 

 lial covering, the tunica propria, and the muscularis mucosse. 



The epithelium lining the lumen of the tube (Fig. 11) is 

 composed of stratified squamous cells, flattened on the surface 

 and gradually becoming thicker and more spheroidal in the 

 lower layers; it is similar to and continuous with the epithe- 

 lium of the pharyngeal mucous membrane. 



It is underlaid by the tunica propria, a dense fibrous layer 

 from the surface of which papillae project upward into the 

 epithelium. 



The muscularis mucosce is a thin layer of involuntary 

 muscle-cells, longitudinally arranged, separating the mucosa 

 from the submucosa. It is not present in the upper part of 

 the oesophagus, but appears below in scattered strands, gradu- 

 ally increases downward, and in the lower part of the tube 

 forms a well-marked complete ring. 



The submucosa, next below, is a layer of loose areolar tissue, 

 which permits the longitudinal folding of the mucous mem- 

 brane. It contains small racemose mucous glands, discharg- 

 ing on the surface, blood and lymphatic vessels, and nerves. 



The muscular layer, which lies beneath or outside of the 

 submucous coat, is composed of two coats, an inner, in which 

 the muscle-cells are arranged circularly or transversely ; and 

 an outer, in which they are longitudinal. In the upper portion 

 of the oesophagus the muscle is striated and voluntary, in the 

 lower portion non-striated and involuntary ; in the middle it 

 is mixed and exhibits a gradual transition from one kind to 

 the other. 



