ORGANS OF DEGLUTITION. 123 



The tongue and cheeks have the power to press back food, 

 that it may pass the isthmus of the fauces, as represented at 

 Fig. 65. 



The pharynx is that part of the alimentary canal which 

 admits of the passage of air from the nasal chamber into the 

 trachea, as it does of food from the mouth to the gullet. 



In the above engraving, it will be seen communicating with 

 the mouth and nose in front, with the eustachian tube going 

 to the ear by a slit, 7, with the gullet marked 5, and with the 

 windpipe, 8. 3 represents a section of the soft palate, which is 

 very long in the horse, and prevents the return of food into the 

 mouth, when it has once passed back through it. Thus, when 

 a horse with a violent sore throat makes a violent effort to 

 drink, and pain prevents the water passing into the gullet, it 

 falls back into the pail through the nose. 



The pharynx is capable of being dilated by three pairs of 

 muscles, and another three pairs act as constrictors. 



In the passage from the mouth into the pharynx, on either 

 side are the tonsils, and the pharynx itself is lined by mucous 

 membrane, which is always moistened, and in the horse espe- 

 cially, by an abundant secretion. 



When not feeding, a horse is observed at intervals to 

 swallow liquid. This secretion has been collected and 

 studied by M. Ricquet. At each of such acts of swallowing, 

 about half an ounce of fluid descends the gullet, and it is 

 found alkaline and very viscid. Ricquet believes that about 

 16 pounds, or 8 kilogrammes daily, are secreted. To prove 

 that the liquid was really from the pharynx and not from the 

 salivary glands, Ricquet opened the ducts of these, and the 

 amount of fluid swallowed continued the same. When food 

 is swallowed by a horse, it is found covered with this T \scid 

 secretion, which Ricquet found alone secreted by the mem- 

 brane just behind the base of the tongue. This abundant 



