DIGESTION 155 



losing its hold on the food, protects it from such injury as might 

 be inflicted by coarse grasses, and is also of value to the animal 

 in cleaning its body. The tongue is supplied with motor power 

 by the hypoglossal nerve and with sensation by the lingual 

 branch of the fifth, which supplies the anterior two-thirds of 

 the mucous membrane, the posterior third being supplied by 

 the Ungual branch of the glossopharyngeal ; the same nerve 

 also supplies the sense of taste to this part of the organ, while 

 taste for the anterior two- thirds is supplied by the chorda 

 tympani of the seventh pair. Division of the hypoglossal nerve 

 in the ruminant prevents the animal from grazing, the dog from 

 lapping, and in all animals the tongue suffers injury by being 

 unable to avoid the incisor teeth. 



The inside of the mouth of the ox is covered with long papillae, 

 which point backwards ; these would appear to be of use in pre- 

 venting the food from falling out of the mouth. In the horse 

 no such papillae exist, in fact the lining membrane of this part 

 is remarkably smooth. The majority of animals have grooves 

 in the palate ; they are well marked in the horse, ox, sheep, and 

 even in the dog. Their function is probably connected with 

 assisting the tongue to pass the food back in the mouth. 



Drinking is performed by the animal drawing the tongue 

 backwards and thus using it as the piston of a suction-pump ; 

 this action produces a vacuum in the front of the mouth, as the 

 result of which the cheeks are drawn inwards, the lips at the 

 same time being closed all round, excepting a small space in 

 front which is placed under water. Such is the method in both 

 horse and ox ; in the former animal the head is extended while 

 drinking, the ears are drawn forward at each swallow, and during 

 the interval fall back. The cause of this motion is not clear, but 

 is probably due to the movement of air in the guttural pouches. 

 A thirsty horse will swallow from 150 to 250 grammes (5 to 8 

 ounces) of fluid at each gulp. Lapping in the dog is performed 

 by curling the tongue in such a way as to convert it into a spoon. 

 Sucking, like drinking, is produced by the animal creating a 

 vacuum in the mouth by closing the lips, decreasing the size of 

 the tongue in front and increasing it behind, the dorsum being 

 applied to the roof of the mouth. The foal places the tongue 

 beneath the nipple and curls it in from each side ; by this means 

 he protects it from the lower incisors and gets a better hold. 



Mastication. 



Mastication is performed between the molar teeth ; the move- 

 ments which the jaws undergo, to admit of this being carried 

 out, depend upon the class of animal. In the dog they are very 



