SPLANCHNOLOGY. 191 



On opening the pharynx from behind, we may observe seven apertures 

 leading from it in different directions : in the upper or nasal portions there 

 are the two posterior nares, and on the side of each of these there is the 

 opening of the Eustachian tube ; below the velum palati there is the isthmus 

 faucium or posterior opening of the mouth ; below and behind the tongue 

 is the opening of the glottis ; and, lastly, we have the termination of the 

 pharynx in the oesophagus. 



7. The CEsophagus is the tube in front of the spine, and behind the 

 trachea, which conducts food from the pharynx to the stomach. When 

 inflated, it is of cylindrical shape, about an inch in diameter, and nine or 

 ten inches long, widening towards the stomach. Like the pharynx, it con- 

 sists of three coats, an external or muscular, a middle or aponeurotic, and 

 an internal or mucous. , 



8. Physiology of Mastication and Deglutition. The first step in the 

 reduction of food taken into the mouth is its mastication, and the addition 

 of salivary matter to the divided particles. Mastication evidently is of great 

 importance in preparing the substances to be afterwards operated on for the 

 action of their solvent ; and it exactly corresponds with the trituration to 

 which the chemist would submit any solid matter, that he might present it 

 in the most advantageous form to a digestive menstruum. The complete 

 disintegration of the alimentary matter, therefore, is of great consequence ; 

 and, if imperfectly effected, the subsequent processes are liable to derange- 

 ment. This derangement we continually meet with ; for there is not, per- 

 haps, a more frequent, source of dyspepsia (difficult digestion) than imperfect 

 mastication, whether resulting from the haste with which food is swallowed. 

 or from the want of the proper instruments. The disintegration of the food, 

 by mechanical reduction, is manifestly aided by insalivation ; it is doubtful, 

 however, to what degree the saliva has any chemical effect upon it. 



When the reduction of the food in the mouth has been sufficiently accom- 

 plished, it is carried into the oesophagus b}^ the action of deglutition. The 

 first stage in the process is the carrying back of the food until it has passed 



■ the anterior palatine arch ; this, which is effected by the approximation of 

 the tongue and palate, is a purely voluntary movement. In the second 

 stage the tongue is carried still further backwards, and the larynx is drawn 

 forwards under its root, so that the e2jigiottis is pressed down over the rima 

 glottidis. The muscles of the anterior palatine arch contract after the 

 morsel has passed it, and assist its passage backwards ; these, with the 

 tongue, cut off completely the communication between the fauces and the 

 mouth. At the same time, the muscles of the posterior palatine arch con- 

 tract in such a manner as to cause the sides of the arch to approach each 

 other like a pair of curtains, so that the passage from the fauces into the 

 posterior nares is nearly closed by them ; to the cleft between the approxi- 

 mated sides the uvula is applied like a valve. A sort of inclined plane, 

 directed obliquely downwards and backwards, is thus formed, and the mor- 

 sel slides along it into the pharynx, which is brought up to receive it. Some 

 of these acts may be performed voluntarily, but the combination of the 



. whole is instinctive. The third stage of the process, the propulsion of the 



ICOSOGRAPHIC KNCYCLOP.EDIA. VOL, II. 57 897 



