90 ANIMAL PHYSIOLOGY. 



pended from this is the larynx, or the upper part of the 

 windpipe, modified as the organ of voice, while to its upper 

 surface is attached the root of the tongue ; and behind it is 

 the epiglottis, a plate of reticular cartilage covered with 

 mucous membrane, which in the state of rest stands straight 

 up in front of the glottis, but in swallowing is folded down 

 over that opening, so as to prevent anything entering into 

 the air passages. 



59. When solid food is taken into the mouth, the teeth, the 

 tongue, the palate, and the muscles of the jaws and cheeks, 

 combine to break it down by mastication. The jaws act as 

 a double lever of the third order, like a pair of tongs ; the 

 powerful muscles which shut them being placed nearer the 

 joints than are the ranges of teeth between which the food 

 is crushed. In biting with the incisor teeth, the jaws are 

 simply brought together ; but in breaking the food up with 

 the molar ranges, the lower jaw is slightly rotated from side 

 to side, at the same time that it is withdrawn from and ap- 

 proached to the upper, and thus the food is rubbed between 

 the opposed cusps as well as bruised by them. The tongue 

 presses the food out from between it and the palate, and 

 throws it on to the teeth on one side or other, thus mixing 

 it up and aiding the thorough permeation of the saliva; and 

 the muscles of the cheeks catch the portions which are 

 crushed outwards by the teeth, and return them inwards. 



The act of swallowing, or deglutition, consists of three 

 parts, of which the first is voluntary, the second spasmodic, 

 and the third involuntary. 



The purely voluntary part consists in pushing the bolus 

 back between the tongue and palate till it reaches the fauces. 

 As soon as it gets there, the will initiates a process, which, 

 when started, proceeds with great rapidity and is finished in 

 a moment, without the will being able to delay it. This 

 spasmodic stage involves a number of structures. If the 

 finger be placed on the hyoid bone, it will be found to be 

 jerked suddenly upwards and forwards; at the same time 

 the back of the tongue, on which the food has been placed, 

 is jerked upwards and backwards; and any one who can 

 succeed in swallowing with the mouth open in front of a 

 looking-glass, will see that the soft palate is made tense, and 



