DEGLUTITION 179 



vascular. It is provided with papillae, some of which are simple conical 

 elevations, while others present two to six conical processes with a sin- 

 gle base. These papillae are rather sparsely distributed over all that 

 portion of the mucous surface which is covered with stratified epithe- 

 lium. 



The contractions of the muscular walls of the pharynx, force the 

 alimentary bolus into the oesophagus, a tube with thick muscular walls, 

 extending to the stomach. The oesophagus is about nine inches (23 

 centimeters) in length. It is cylindrical and slightly constricted at its 

 superior and inferior extremities. Its upper extremity is in the median 

 line behind the lower border of the cricoid cartilage and opposite the 

 fifth cervical vertebra. At first, as it descends, it passes a little to the 

 left of the cervical vertebrae. It then passes from left to right from 

 the fourth or fifth to the ninth dorsal vertebra, to give place to the aorta. 

 It finally passes a little to the left again, and from behind forward, to its 

 opening into the stomach. In its passage through the diaphragm, it is 

 surrounded by muscular fibres, so that when this muscle is contracted in 

 inspiration, its action has a tendency to constrict the opening. 



The coats of the oesophagus are two in number, unless there be 

 included, as a third coat, the fibrous tissue which attaches the mucous 

 membrane to the subjacent muscular coat. 



The external coat is composed of an external longitudinal, and an 

 internal circular, or transverse layer of muscular fibres. In the superior 

 portion, the longitudinal fibres are arranged in three distinct fasciculi ; 

 one in front, which passes downward from the posterior surface of the 

 cricoid cartilage, and one on either side, extending from the inferior con- 

 strictors of the pharynx. As the fibres descend, the fasciculi become 

 less distinct and finally form a uniform layer. The circular layer is 

 somewhat thinner than the external layer. Its fibres are transverse 

 near the superior and inferior extremities of the tube and are somewhat 

 oblique in the intermediate portion. The muscular coat is -g 1 ^ to y 1 ^ of 

 an inch (0.5 to 2.1 millimeters) in thickness. 



In the upper third of the oesophagus, the muscular fibres are of the 

 red or striated variety, with some anastomosing bundles ; but lower down, 

 there is a mixture of non-striated fibres, which appear first in the circular 

 layer. These latter fibres become gradually more abundant, until, in the 

 lower fourth, they largely predominate. A few striated fibres, however, 

 are found as low down as the diaphragm. 



The mucous membrane of the oesophagus is attached to the mus- 

 cular tissue by a dense fibrous layer. It is quite vascular and reddish 

 above, but gradually becomes paler in the inferior portion. The mucous 

 membrane ordinarily is thrown into longitudinal folds, which are obliter 



