188 DIGESTION. [CHAP. xxm. 



brane in the vicinity. The pellet of food having arrived near the 

 oesophagus, is projected into it by the contraction of the middle 

 and inferior constrictors, the upper portion of that canal being 

 dilated by its entrance. 



It might be imagined that a process which may thus be artificially 

 divided into consecutive stages, and which combines so many ela- 

 borate and harmonized actions, would occupy something more than 

 a single second in its performance. It is, however, quite momentary. 

 The movements cannot be performed separately by any voluntary 

 control ; the food once willingly thrown by the tongue beyond the 

 isthmus of the fauces cannot be recalled, but is necessarily carried 

 forward to the stomach a beneficent provision, in which the phy- 

 sical supersede in a great degree the mental nervous actions, to 

 ensure the integrity of the vital function of respiration. On the 

 action of the nerves, however, we need add nothing to what has been 

 already stated (see vol. i. pp. 333, 345, 346; and vol. ii. pp. 117, 120). 



The oesophagus is a tube continuous with the pharynx, fitted to 

 convey the food past the organs of respiration and circulation in 

 the thorax to the stomach below the diaphragm. It first lies upon 

 the vertebrae and inclines slightly to the left, but afterwards, in its 

 course through the posterior mediastinum, it advances in front of 

 the descending aorta, and occupies the median line. It is sur- 

 rounded in its whole length by a lax areolar tissue, which permits 

 its dilatation and contraction during the passage of the food. It 

 has a strong muscular coat composed of two layers, an outer, of 

 longitudinal fibres, which commence from the cricoid cartilage; and 

 an inner, of circular fibres : both these spread out upon the stomach, 

 and become much thinner on that organ. The fibres of both layers 

 are of the striped kind in the upper part, and for a variable way 

 down, some being often traceable as low as the diaphragm. In the 

 middle region, unstriped fibres are mingled with the others ; and in 

 the lower part they are either the chief or only constituent. It has 

 also a mucous lining, continuous with that of the pharynx and sto- 

 mach, but different from both, being covered with a thicker and 

 more opaque epithelium, resembling cuticle, and thrown, when 

 empty, into longitudinal folds by the help of an abundant areolar 

 tissue between the coats. Among the creases, chiefly in the lower 

 third, are scattered mucous glands, which open on the surface, and 

 serve to lubricate the canal during the passage of food. The cuti- 

 cular lining of the oesophagus is changed abruptly at the cardiac 

 orifice of the stomach into the glandular lining of the latter organ. 

 Thus the resophagus is organized as a simple conduit. It has con- 



