No. 2.] THE HARD PARTS OF THE MAMMALIA. 265 
to flatten it out against the otic bulla, and atrophy it. The 
lower jaw would thus come to occupy that peculiarly posterior 
position which it does in all rodents. 
The anteroposterior (proal!) type of mastication becoming 
necessary, an appropriate development of the muscles moving 
the lower jaw, with their insertions, follows, pavz 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 up- 
wards, a movement which is commenced so soon as the inferior 
incisor strikes the apex of the superior 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 coronoid 
process of the mandible to have become gradually reduced in 
size to complete disappearance in some of the genera, e.g. of 
Leporidz. In these genera 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 internal pterygoid 
and masseter muscles and their insertions and origins. The 
angle of the ramus being forced backwards, these muscles are 
gradually stretched backwards at their insertions, and their con- 
traction becomes 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 mandible becomes more or less 
inflected. In their effect on the movements of the ramus they 
oppose that of the temporal muscle, since they draw the ramus 
forwards. They are the effective muscles in the use of the 
incisor teeth; that is, in the opposition of the inferior incisors 
against the superior from below and posteriorly. Hence the 
great development of the internal pterygoid and, in a less degree, 
of the masseter. 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 
1 See page 226 for explanation of the different modes of mastication. 
