348 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



out against the otic bulla, and atrophy it. The lower 

 jaw would thus come to occupy that peculiarly pos- 

 terior position which it does in all rodents. 



The anteroposterior (proaP) type of mastication 

 becoming necessary, an appropriate development of 

 the muscles moving the lower jaw, with their inser- 

 tions, follows, pan passu. As a result we see that the 

 insertion of the temporal muscle creeps forward on the 

 ramus, until in the highest rodents (Cavia) it extends 

 along the ramus to opposite the first true molars. The 

 office of this muscle is to draw the ramus backwards 

 and upwards, a movement which is commenced so 

 soon as the inferior incisor strikes the apex of the su- 

 perior incisor on the posterior side. By this muscle 

 the inferior molars are drawn posteriorly and in close 

 apposition to the superior molars. Connected with 

 this movement, probably as an effect, we find the co- 

 ronoid process of the mandible to have become grad- 

 ually reduced in size to complete disappearance in 

 some of the genera, e. g. of Leporidffi. In these gen- 

 era the groove-like insertion of the temporal muscle 

 develops as the coronoid process disappears. 



As third and fourth effects of the posterior position 

 of the lower jaw, we have the development of the in- 

 ternal pterygoid and masseter muscles and their inser- 

 tions and origins. The angle of the ramus being forced 

 backwards, these muscles are gradually stretched back- 

 wards at their insertions, and their contraction be- 

 comes more anteroposterior in direction than before. 

 The internal pterygoid becomes especially developed, 

 and its point of origin, the pterygoid fossa, becomes 

 much enlarged. The border of the angle of the man- 

 dible becomes more or less inflected. In their effect 



IPage 318. 



