(ESOPHAGUS. 299 



then passes lo tlio upper bide of the trachea, over the base of 

 the heart, through the posterior mediastinum, and through the 

 diaphragm by the foramen sinistrum; it thus gains the abdom- 

 inal cavity, and terminates in the cardiac orifice of the stomach. 

 It is narrowest at its origin behind the larynx, then of uniform 

 size until it reaches the diaphragm", where it again constricts. 



The oesophagus may be said to consist of a cervical, a thoracic, 

 and an abdominal portion, the latter being very short. It con- 

 sists of two tunics, an internal mucous and an external muscular, 

 which are very loosely united by areolar tissue. The mucous 

 coat is continuous with that of the pharynx and stomach, but is 

 pale in colour, and clothed with dense stratified epithelium ; it 

 presents numerous longitudinal folds, which allow of considerable 

 dilatation, and a number of racemose glands. The muscular 

 coat consists of an external longitudinal, and an internal circular 

 layer of fibres. The cervical and anterior thoracic muscular coat 

 :! striated, the remainder non-striated, but both are involuntary. 

 The longitudinal layer commences by three fasciculi, one in front 

 and one on either side, which unite to form a continuous layer. 

 The circular fibres originate at the inferior constrictors of the 

 pharynx. Posteriorly, both sets of fibres pass to the muscular 

 coats of the stomach. 



The oesophagus is supplied with blood anteriorly by twigs 

 from the carotid, and the broncho-cesophageal arteries posteriorly; 

 its nerves come from the pneumogastric. The oesophagus has 

 no other use than for the conveyance of aliment from the 

 pharynx to the stomach. Its most noteworthy features in the 

 horse are, — first, the abrupt manner in which it enters the 

 stomach, hardly dilating at all before doing so ; secondly, the 

 enormous proportionate size of its mucous membrane posteriorly ; 

 this is so large, that near the stomach its folds completely fill 

 the tube when empty, thus acting as a valve, and preventing 

 regurgitation of the food. 



The process of deglutition, or swallowing the food, is performed 

 in the following manner : — The masticated food gathers as a 

 bolus at the root of the tongue, which by retracting forces the 

 pellet through the isthmus faucium into the pharynx ; the velum 

 pendulum palati recedes backwards and upwards, while the food 

 forces down the epiglottis, effectually closing the larynx; finally, 

 the pharynx, by its constrictor muscles, grasps the bolus, a-nd 

 presses it downwards into the oesophagus. » The passage of food 



