DEGLUTITION. 205 



vided with ciliated, columnar epithelium, like that which covers the mem- 

 brane of the posterior nares. Laterally, below the level of the opening of the 

 Eustachian tubes, and posteriorly, at the point where it becomes vertical, the 

 mucous membrane abruptly changes its character. The epithelial covering 

 is here composed of flattened cells, similar to those which cover the mucous 

 membrane of the oesophagus. The membrane is also paler and less vascular. 

 It is provided with papillae, some of which are simple, conical elevations, while 

 others present two to six conical processes with a single base. These papillae 

 are rather thinly distributed over all of that portion of the mucous surface 

 which is covered with flattened epithelium. 



The contractions of the muscular walls of the pharynx force the aliment- 

 ary bolus into the oesophagus, a tube possessed of thick, muscular walls, ex- 

 tending to the stomach. The oesophagus is about nine inches (23 centi- 

 metres) in length. It is cylindrical and is 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 verte- 

 bra. 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 close 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 tissue. 



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

 nal 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 froni the posterior surface of the cricoid cartilage, 

 and one on either side, extending from the inferior constrictors of the pharynx. 

 As the fibres descend, the fasciculi become less distinct and are finally blended 

 into a uniform layer. The circular layer is somewhat thinner than the ex- 

 ternal layer. Its fibres are transverse near the superior and inferior extrem- 

 ities of the tube and are somewhat oblique in the intermediate portion. The 

 muscular coat is -fa to ^ of an inch (0*5 to 2*1 mm.) in thickness. 



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

 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 cir- 

 cular 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 muscular 

 tissue by a dense, fibrous layer. It is quite vascular and reddish above, but 

 gradually becomes paler in the inferior portion. The mucous membrane is 

 ordinarily thrown into longitudinal folds, which are obliterated when the 



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