ANATOiVIY OF THE HORSE. 
151 
ciliated and squamous, excepting at the posterior part, where it 
is joined to the petrous portion. The superior part lies upon 
the side of the parietal bone, and behind that upon a very small 
proportion of the occipital ; in front, it lies upon the frontal, 
and below upon the sphenoid. 
THE PETROUS PORTION. (PARS PETROSA.) 
Is important from lodging the internal organ of hearing. It 
presents two parts perfectly distinct from each other in structure, 
position, and use ; one is external or mastoideal, the other inter¬ 
nal or cerebral. From the former projects downwards the posterior 
mammiform or mastoid process, in which is distinguished a base 
inferiorly, and a crest superiorly: the latter joins the occipital 
crest. Below and rather towards the front of the process is the 
external orifice of the aqueduct of the vestibule; directly in 
front of the process is the external auditory meatus and 
foramen, surrounded by a jutting oval rim, notched in the 
middle, which constitutes the auditory process ; to the inner 
side of the meatus is the hyoideal process, surrounded by 
a prominent circular edge, with which is connected the 
os hyoides. Behind the meatus is a general protuberance, 
spheroid without, cavernous within, bottomed by a thin bony 
shell, within which are inclosed the mastoid cells ,* proceeding 
from the inferior, anterior, and inner part of the bone is the 
styloid process ; at whose root are two openings, one below for 
the lustachian tube, the other above, the styloid foramen, for 
the chorda tympani. 
J'he Internal Part exhibits three surfaces for consideration. 
One superior and posterior, smooth though uneven, is excavated 
for adaptation to the side of the cerebellum, by which it is im¬ 
pressed in places; it has a prominence running across its 
middle, and just beneath this an irregular aperture having 
interiorly two distinct passages, one terminated by a cul-de-sac, 
including a foramen, which leads to the cochlea and communi¬ 
cates through some small pores with the labyrinth; the other, 
the internal auditory foramen, is for the conduit of the auditory 
nerves, and communicates with the aqueduct of the vestibule. 
Another surface, the anterior, contributes a little to the forma¬ 
tion of the concavity for the posterior lobe of the cerebrum. The 
inferior surface, parted from the superior by a fissure, is convex 
and prominent, but irregularly so : it constitutes the exterior 
wall of the labyrinth. 
The petrous portion is received between the squamous and 
the occipital bone, . and though it is rarely found united with 
