376 THE DIGESTIVE APPARATUS IN MAMMALIA. 



pliaryngeal and thyroideal arteries. The nerves are supplied by the glosso- 

 pharyngeus, pneumogastric, and great sympathetic. 



FUNCTIONS. The pharynx plays a passive part in respiration, by serving 

 as an intermediate canal between the nasal passages and the larynx. 



Its principal function, however, is connected with the digestive pheno- 

 mena, by its being an active agent in the first stage of deglutition a complex 

 and rapid movement, which is executed in the following manner : The bolus of 

 food, propelled by the tongue into the pharynx, is seized by the constrictor 

 muscles, which come into action successively from before to behind, in a 

 peristaltic and involuntary manner, to carry the mass to the entrance of 

 the oesophagus. The food thus passes over the opening of the larynx during 

 pliaryngeal deglutition, but it cannot enter it, because the bolus forces back 

 the opiglottis on this aperture, which it almost exactly closes ; because, also, 

 the passage of the food prevents pulmonary inspiration, which might, if 

 allowed to take place, divert it from its natural course, and throw it into 

 the nasal air-passages ; the application of the walls of the pharynx to the 

 pellet of food during its momentary passage over the larynx, intercepts all 

 communication between the external air and the lungs, and only permits 

 the elevation of the ribs with the utmost difficulty. .The extreme rapidity 

 of the act of deglutition is another reason for the food escaping the larynx. 



(Gray concisely remarks : When deglutition is about to be performed, 

 the pharynx is drawn upwards and dilated in different directions to receive 

 the morsel propelled into it from the mouth. The stylo-pharyngei, which 

 are much further removed from one another at their origin than at their 

 insertion, draw upwards and outwards the sides of this cavity, the breadth 

 of the pharynx, in the antero-posterior direction, being increased by the 

 larynx and tongue being carried forwards in their ascent. As soon as the 

 morsel is received in the pharynx, the elevator muscles relax, the bag 

 descends, and the constrictors contract upon the morsel and convey it 

 gradually downwards into the oesophagus.) 



The deglutition of liquids is carried on in a similar manner. 



It is curious to remark that, in Solipeds, the food does not come into 

 direct contact with the greater portion of the superior wall of the pharynx 

 during its passage through that cavity. When the alimentary mass is 

 carried back by the tongue, it raises the soft palate and bears its posterior 

 border backwards to the entrance of the oesophagus. The extreme develop- 

 ment of this palatine curtain, therefore, quite prevents this surface of the 

 pharynx from being directly applied to the food, and it is through the medium 

 of this partition that the constrictors exercise their peristaltic action on the 

 morsel of aliment until it reaches the cesophageal infundibulum. 



DIFFERENTIAL CHARACTERS OF THE PHARYNX IN OTHER THAN SOLIPED ANIMALS. 



The pharynx of Ruminants is long and very spacious. The hyo-, thyro-, and crico- 

 pharyngeal muscles the first and second of the two middle constrictors, and the inferior 

 constrictor are less distinct from each other than in the Horse ; the last-named muscle 

 is very small, and the fibrous raphe on which the constrictors unite is little developed. 

 In the pharynx of the Sheep is noticed a mucous duplicature that descends to the middle 

 of the posterior wall, and appears to be a continuation, posteriorly and inferiorly, of the 

 nasal septum. 



In the Pig, the posterior part of the pharynx is narrow, and has a pouch immediately 

 above tho glottis, between the thyro- and crico-pharyngeal muscles. 



In the Dog, the infundibulum is very spacious, and the pliaryngeal mucous membrane, 

 much finer than that of the ossophagus, is distinguished from it by a well-marked line 

 of demarcation. The crico-pharyngeal is not very distinct from the thyro-pharyiigeal 

 muscle ; so that, in reality, only three constrictors can be distinguished. 



