Digestion. 97 



other. The papiihe of the tongue will be dealt with in 

 speaking of Taste. 



The prehension of liquids is caused by the formation of a 

 vacuum in the mouth by the pumping action of the tongue, 

 whilst the lips are kept tightly sealed, excepting an opening- 

 left in the front which is kept below the level of the water. 

 At each swallow the ears are drawn forwards, and during 

 the interval they fall backwards. 



During Mastication certain movements of the jaws are 

 performed, to admit of which the articulation of the upper 

 and lower jaw is exceedingly large. The disc of cartilage 

 placed here between the two bones is to save the parts from 

 injury, and to admit of the extensive lateral or rotatory 

 motion so marked in the herbivora. 



It is singular to observe that the upper jaw in the horse 

 is so much wider than the lower one ; the upper molar teeth 

 are also much larger. Owing to this, the tables of the 

 teeth instead of wearing with a level surface, present a very 

 oblique and chisel-shaped arrangement, the upper teeth 

 being sharp on the outside, the lower row of teeth on the 

 inside. This is an important clinical fact, sharp teeth 

 giving rise to much trouble and suffering. 



Mastication in both horse and ox is lateral and some- 

 what rotatory ; that is to say, after the teeth have crossed 

 each other from left to right, or vice versa, the mouth is 

 opened, and the molars brought back to their original 

 position. In the horse the lateral movement occurs from 

 left to right, or from right to left, never from rear to front, 

 or front to rear. Mastication occurs on one side only, and 

 when that gets tired the food is passed to the other. One 

 side takes a long time to tire ; mastication may be per- 

 formed for a whole hour in one direction. 



In the peculiar lateral movement of the jaws of the horse 

 and ox, the changes which occur in the articulation have 

 been pointed out by Gamgee* to be as follows : During the 

 rotatory movement, or lateral displacement, one of the 



* 'Our Domestic Animals in Health and Disease.' 



7 



