218 DIGESTION. 



brane of the posterior nares. It is marked by a deep antero-posterior groove in the 

 median line ; and, on either side, parallel with the median line, are four smaller grooves. 

 In the horizontal portions, the mucous membrane in the central groove adheres to the 

 periosteum of the basilar process, particularly at its posterior extremity. Laterally, be- 

 low 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 cells of the pavement-variety, similar to those which cover 

 the mucous membrane of the oesophagus. The membrane is also paler and is less rich 

 in blood-vessels. It is provided with papillae, some of which are simple, conical eleva- 

 tions, while others present from 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 pavement-epithelium. 



The contractions of the muscular walls of the pharynx force the alimentary bolus into 

 the oesophagus, a tube possessed of thic^mjis^ula^walls^ extending to the stomach. The 

 oesophagus is about ^in^jn^]^sjii_length^. It is Cylindrical, and rather constricted at its 

 ^superior and mferiox^ejtomitigs. It commences in the niedian line_ behind the lower 

 border of the cricoid cartilage and opposite the ^fth^eimc^ljrej^tebra. At first, as it de- 

 scends, it pasjiejtsJMtleutQ thgj^t^of the cervical vertebra). 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 byjmigcujar^ 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 we include, 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 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 pos- 

 terior 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 external 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 from ^ to T ^ of an inch 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 circular layer. These latter fibres become 

 gradually more numerous, 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 becomes gradually paler in the 

 inferior portion. The mucous membrane is ordinarily thrown into longitudinal folds, 

 which are obliterated when the tube is distended. Its epithelium is thick, of the pave- 

 ment-variety, and is continuous with, and similar to the covering of the lower portion of 

 the pharynx. It is provided with papillae of the same structure as those found in the 

 pharynx, the conical variety predominating. Numerous small, racemose glands are found 

 throughout the tube, forming by their aggregation at the lower extremity, just before it 

 opens into the stomach, a glandular ring. 



Mechanism of Deglutition. For convenience of description, physiologists have gen- 

 erally divided the process of deglutition into three periods. The first period is occupied 

 by the passage of the alimentary bolus backward to the isthmus of the fauces. This may 

 appropriately be considered as a distinct period, because the movements are effected by 



