154 A MANUAL OF VETERINARY PHYSIOLOGY 



to inflict on grasses and grain. The teeth in herbivora, both 

 incisors and molars, are constantly, though slowly, being pushed 

 out of the sockets which hold them ; in this way wear and tear 

 is compensated for, whilst the fang of the tooth becomes corre- 

 spondingly reduced in length. It is owing to this fact that the 

 incisor teeth alter in shape and direction, and so enable the age 

 to be determined. The tables of the molar teeth are not flat 

 but oblique ; this is especially well seen in the horse where the 

 cutting surface is chisel-shaped, the upper teeth being longest on 

 the outside, while those of the lower row are longest on the inside 

 (see Fig. 57). This arrangement produces sharp teeth, which 

 are a constant source of trouble and loss of condition in horses. 



Fig. 57. — Schematic Transverse Section of the Upper and Lower Jaws 

 of the Horse between the Third and Fourth Molars, showing the 

 Position of the Tables of the Teeth during Rest and Mastication. 



UJ, Upper jaw ; LJ, lower jaw ; RM, right molar ; LM, left molar ; RLM, right 

 lower molar ; LLM, left lower molar, i, The position of the teeth during 

 rest, the outside edge of the lower row in apposition with the inside edge of 

 the upper. 2, The jaws fully crossed masticating from left to right ; the 

 • tables of both upper and lower molars now rest on each other. 3, The 

 position halfway through the act of mastication ; the outer half of the 

 lower teeth wearing against the inner half of the upper. 



The movements of the tongue are important. In the ox and 

 dog they are very extensive, the former animal having no difficulty 

 in protruding the tongue and even introducing the tip into the 

 nostrils. It is not a very common habit with horses to protrude 

 the tongue except when yawning, but they have considerable 

 power in withdrawing it in the mouth. A great difference exists 

 between the tongue of the horse and that of the ox ; the former 

 is flabby, broad and flat at the end, constricted opposite the 

 frenum, and swelling out at the apex ; it is comparatively smooth 

 on its surface. The tongue of the ox narrows from base to apex, 

 the latter being pointed ; it is very rough, which prevents it from 



