THE DIGESTIVE SYSTEM 45 



is dorsal to the trachea, but soon it deviates 

 towards the right of the middle line of the body. 

 At the thoracic entrance a considerable dilata- 

 tion, the crop, lies to the right. Beyond this 

 the oesophagus narrows somewhat before its 

 mergence into the stomach. 



Structurally, the wall of the oesophagus is 

 composed of four layers of tissue. The outer- 

 most is a loose and rather sparse fibrous invest- 

 ment, which serves to connect the tube with its 

 immediate surroundings. The next tunic con- 

 tains muscular tissue, and may be divided 

 into two strata (Fig. 13). The outer stratum 

 contains muscular fibres running for the most 

 part longitudinally. The fibres of the inner 

 stratum are circular. A loose layer of sub- 

 mucous tissue joins the innermost tunic, the 

 mucous membrane, and the muscular layer. 

 The mucous membrane is of considerable 

 thickness, and, when the oesophagus is empty, 

 is thrown into longitudinal folds. The epi- 

 thelium is thick and composed of many layers 

 of cells, of which the most superficial are 

 flattened and horny. Numerous conspicuous 

 alveolar glands occupy the tissue underlying 

 the epithelium, and are provided with ducts 

 which open on the free surface of the mucous 



