418 



MAMMALIAN ANATOMY 



FIG. 338. 



internal lower teeth come between the crowns of the two internal 

 upper ; the middle lower on each side comes between the internal and 

 middle upper, and the external lower between the middle and external 

 upper. The external upper incisor, therefore, is between the external 

 lower incisor and the lower canine. 



The incisors and canines are used for grasping and biting food ; 

 cutting is done by the posterior teeth, especially by the sectorials (Fig. 



336), near which are inserted the powerful 

 muscles of mastication. The lower back 

 teeth are nearer the middle line of the 

 mouth than are the upper teeth, which 

 they do not touch when the jaws are 

 moved in an absolutely vertical line ; 

 hence when the cat chews on one side of 

 the mouth the lower jaw must be brought 

 to that side, that the outer surface of the 

 lower teeth may shear upward and for- 

 ward against the inner surface of the 

 upper teeth. If, however, the mass of 

 food is very large, we may suppose that 

 at first the lower jaw will be thrown to 

 the opposite side, where the lower teeth 

 will grind directly against the upper. 

 The areas of wear on the back teeth 

 therefore appear to be produced in several 



wa yg : namely, by the ojrilldino- Q f } jarc [ 



su b s tances between the teeth, the nearest 

 surfaces of opposite teeth suffering first ; 



by the wearing of tooth on tooth when softer substances are chewed ; 

 and by the wearing of tooth on tooth on the side of the jaw not 

 actively in use in chewing. Inasmuch as the lower jaw articulates 

 with the skull by means of a hinge-joint, the larger posterior teeth 

 come together before the smaller anterior teeth. 



The relation between the upper and lower back teeth is shown in 

 Fig. 337 ; it may be also represented by a diagram (Fig. 338) which 

 exhibits the teeth as viewed from below when the lower teeth are 

 supposed to be transparent. The heavy lines are the outlines of the 

 lower teeth, the light lines the outlines of the upper teeth. 



DIAGRAM OF APPOSITION OF POS- 

 TERIOR TEETH. 



a, anterior cusp ; b, middle cusp ; c, posterior 

 cusp; d, talon. Heavy outlines show 

 lower teeth ; light outlines show upper 

 teeth. 



