KINETOGENESIS. 349 



on the movements of the ramus they oppose that of 

 the temporal muscle, since they draw the ramus for- 

 wards. They are the effective muscles in the use of 

 the incisor teeth ; that is, in the opposition of the in- 

 ferior incisors against the superior from below and 

 posteriorly. Hence the great development of the in- 

 ternal pterygoid, and, in a less degree, of the masse- 

 ter. Both muscles tend also to close the jaws, but at 

 a different point in the act of mastication from that at 

 which the temporal acts. If we suppose the mouth 

 to be open, the action of the masseter and internal 

 pterygoid muscles draws the mandible forwards and 

 upwards until the incisors have performed their ofHce, 

 or the molars are in contact with each other or with the 

 food. They then relax, and their temporal muscle 

 continues the upward pressure, but draws the ramus 

 backwards to the limit set by the adjacent parts, caus- 

 ing the act of mastication. 



A fifth effect of the development of the incisors and 

 of the proal mastication, is seen in the position of the 

 molar teeth. The indefinitely repeated strain and 

 pressure applied to the superior molars from forwards 

 and below has evidently caused a gradual extension of 

 the maxillary bone backwards, so that the last molars 

 occupy a position much posterior to that which they 

 do in other orders of mammals. This is especially the 

 case in such forms as Bathyergus, Arvicola, and Cas- 

 toroides (Figs. 107-108), where the last molars are be- 

 low the temporal fossa, and posterior to the orbit. 



A sixth effect of the causes mentioned has been re-- 

 f erred to by Ryder. ^ This is the oblique direction of 

 the axes of the molar teeth. These directions are op- 

 posite in the two jaws ; upwards and forwards for the 



\ Proceedings Philadelphia Academy, 1877, p. 314. 



