THE SENSE CAPSULES. 25 
squamosal. They are divided into two 
portions: a petrous portion, arising 
from the pro- and opisthotic; and a 
mastoid portion, arising from the 
epiotic. 
The petrous portion forms the anterior 
part, and encloses the internal ear. 
On its outer surface are two foramina 
situated almost above each other: the 
larger is the fenestra ovalis, and the 
smaller the fenestra rotunda; to- 
gether they form the means of com- 
munication between the internal ear, 
and the tympanic cavity. On the 
inner surface of the petrous portion 
of the periotic, there are two large 
fossa: the upper and larger one is the 
floccular fossa’, which lodges the 
floccular lobe of the cerebellum ; the 
lower one has a ridge of bone passing 
across it, which divides it into an 
anterior and a_ posterior opening. 
The posterior one is the meatus 
auditorius internus, which transmits 
the auditory nerve to the internal 
ear. The anterior opening forms the 
? Its size varies in different skulls. In many mammals it is entirely 
absent (e.¢. Sheep, Elephant, Zchzdiza &c.) In some of the Rodentia 
(Porcupine, Capybara) it is also absent, while in others (Beever, 
Chinchella) it is very conspicuous. 
