6^ THE PERMANENT DENTITION. 



arch,* is greater in the region of the first grinders 

 than in that of the sixth. Therefore it will be per- 

 ceived that it is only alternately that the rows are used 

 in the performance of the masticatory function, and 

 that were the grinders in exact apposition (edge to 

 edge), the lateral and semicircular movement of the 

 lower jaw would be as awkward and unnatural in the 

 case of the horse as the same movement would be in a 

 human being. 



There are still other contrasts between the grinders. 

 According to Surgeons M. H. Bouley and P. B. Fergu- 

 son, the upper teeth are slightly convex, the lower 

 slightly concave. Again, according to Charles D. 

 House, the outer surface of the upper grinders is pro- 

 vided with a coat of enamel twice as thick as that of 

 the inner, while the , reverse is the case with the lower 

 teeth. There is design in this provision of Nature 

 (notwithstanding Mr, House says it is inexplicable), 

 for the projecting edges receive that which they re- 

 quire, to wit, strength in proportion to their hight; 

 otherwise they would be easily broken off. As the 



* Prof. Youatt says : " The brandies of tlie lower jaw termi- 

 nate in two processes, tlie coracoid (beak-like), and the condy- 

 loid (rounded). The coracoid passes under the zygomatic arcli, 

 the temporal muscle being inserted into it and wrapped round 

 it. The condyloid is received into the glenoid (shallow) cavity 

 of the temporal bone, at the base of the zygomatic arch, and 

 forms the joint on which the lower jaw moves. The joint ad- 

 mits of a hinge-like motion, which is the action of the jaw in 

 m]>piiig the herbage and seizing the corn. The corn, however, 

 must be ground ; bruising and champing it are not sufficient for 

 the purposes of digestion. It must be put into a mill. It is put 

 into a mill, and as perfect a one as imagination can conceive. 

 The construction of the glenoid cavity gives the required lateral 

 or grinding motion." 



