261 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



3. The tonuue, lips, and cheeks are accessory organs of mastication, 

 and act b} r maintaining the food in its position between the teeth. In 

 man}' animals, particularly the horse, as already mentioned, the lips have 

 a high degree of prehensile power, and when the upper lip is paralyzed, 

 as by section of its motor nerve, prehension of food in the horse is 

 impossible. When the lips and cheeks are paralyzed, again, even in 

 animals in which these organs do not serve for prehensile purposes, mas- 

 tication is impossible, or at least rendered extremely difficult, from the 

 fact that their loss of motor power prevents their keeping the food 

 between the teeth ; their loss of sensibility prevents the determination 

 as to when the food has acquired the proper degree of comminution, and 

 their insensibility prevents these organs avoiding being themselves 

 lacerated by the teeth. This especially applies to the occurrence of mas- 

 tication between the molar teeth. Here, when the buccinator muscles 

 are paralyzed, through paralysis of the facial nerve (seventh pair), the 

 food collects in the pouches formed by the relaxed cheeks, and cannot be 

 properly masticated. The sensibility of the lips and cheek is derived 

 from the fifth pair of nerves. 



The tongue is also an important organ of mastication ; it serves in 

 a large group of animals for the prehension of solids and liquids ; it is 

 of great importance in starting the process of deglutition, and in man it 

 is one of the principal organs of articulation. By its high degree of 

 sensibility it aids in mastication in determining the degree of comminu- 

 tion of the food, and in keeping the particles of food between the teeth 

 during their mastication ; its mobility enables it to act as a sort of hand 

 in the necessary movements of the bolus of food in the mouth. Its 

 muscles are striped and voluntary in nature and arranged in four different 

 layers ; an upper and lower layer, passing from the root to the tip of the 

 tongue, and an upper and lower oblique layer. These muscles form a 

 complicated net-work of fibres which, by varying degrees of partial con- 

 traction, permit not only changes in the shape of the tongue, but also in 

 its position within and without the mouth. The extension of the tongue 

 is accomplished by the muscles passing from the chin to the body of the 

 organ, the genio-glossus muscles. The retraction of the tongue is accom- 

 plished by means of the muscles arising from the hyoid bone and styloid 

 process, — the hyo- and stylo-glossus muscles, — and by the longitudinal 

 fibres in the body of the tongue. The different alterations in shape of 

 the tongue are accomplished by the contraction of its intrinsic muscles. 

 Thus, when the upper longitudinal fibres of the tongue contract the tip 

 of the tongue is elevated. When those of the inferior layer contract the 

 tip of the tongue is depressed, and by contraction of the upper oblique 

 layers the tip of the tongue is formed into the spoon shape which is so 

 useful in the prehension of liquids in the cat tribe. The motor power 



