DIGESTIVE SYSTEM. 241 



in colour, and is papillary and in places rugose upon its surface; 

 being perforated by the ducts of numerous follicles which dis- 

 charge a mucus that preserves glibness and moisture to its 

 interior. The membrane itself is (where it meets them) con- 

 tinuous both with the buccal membrane and that which lines the 

 esophagus. 



Although the pharynx is designed for the reception of the 

 food, it does not open directly into the mouth: the two cavities 

 are separated from each other by the soft palate and epiglottis. 

 Except in the act of swallowing and coughing, they have no com- 

 munication : in the former case, the velum is pressed upward by 

 the food against the posterior openings of the nose ; in the latter, 

 the larynx is depressed by a convulsive action of the muscles in 

 the vicinity. Into the cavity above the velum there are four 

 openings — two of the chambers of the nose, one of the larynx, 

 and one of the esophagus : the eustachian tubes do not open 

 into the pharynx ; they end in two large membranous pouches at 

 the upper part of the fauces. The opening leading into the 

 esophagus is constantly closed, except when alimentary matters 

 are passing to or from the stomach ; so that air received into the 

 pharynx through the nose can pass nowhere else but into the 

 windpipe ; but if food be returned from the stomach, it will be 

 regurgitated into the nose; at least, only that portion of it which 

 enlers the pharynx at the moment that the larynx is depressed 

 in the act of vomiting, can be thrown into the mouth : in the 

 same way that air is in the act of coughing. 



OF THE ESOPHAGUS. 



The esophagus, or gullet, is the tube through which the food is 

 conducted from the pharynx into the stomach. 



Course. — It has its beginning from the pharynx, and is there 

 placed at the upper and back part of the larynx, taking the first 

 part of its course above and behind the trachea, between that 

 tube and the cervical vertebrae. Having proceeded a short way 

 down, it inclines to the left, and soon after makes its appearance 

 altogether on the left side of the trachea, and continues so placed 

 during the remainder of its passage down the neck : this explains 

 why we look for the bolus during the act of swallowing on the 

 left, and not on the right side of the animal. In company with 

 the trachea, the esophagus enters the thorax between the first 

 two ribs, at which place, running above that tube, it quits its com- 

 panion for the superior mediastinum, which cavity it traverses 

 below and a little to the right of the posterior aorta. Imme- 

 diately beneath the decussation of the crura, the esophagus 

 1 i 



