62 THE SKELETON OF THE HORSE 
and the root of the paramastoid process of the occipital bone. It is crossed by a 
groove which leads to the mastoid foramen (Foramen mastoideum), from which a 
canal extends forward to the temporal canal. 
The medial surface faces into the cerebellar fossa of the cranium. It is con- 
cave and smooth, but irregular. In its ventral part is the entrance to a short canal, 
the internal acoustic meatus (Meatus acusticus internus), which transmits the 
seventh and eighth cranial nerves. 
The entrance to the meatus is termed the porus acusticus internus. The fundus of the 
meatus is divided by a crest into two fossw. In the superior one is the origin of the facial canal, 
which curves through the bone and opens externally at the stylo-mastoid foramen; it transmits 
the facial (seventh cranial) nerve. The inferior fossa presents small foramina for the passage of 
fibers of the acoustic (eighth cranial) nerve. 
Behind the meatus and near the posterior margin of the surface is the slit-like 
external opening of the aquieductus vestibuli (Apertura externa aqueeductus ves- 
4 3 1 
2 10 9 8 
Fic. 40.—Lerr Perrovus Temporat BoNE oF Horse; Fic. 41.—Lerr Perrovs Temporal BONE OF HORSE; 
LareraL VIEW. PoOSTERO-MEDIAL VIEW. 
1, External acoustic meatus; 2, mastoid proc 3, 1, Mastoid process; 2, notch which concurs with 
hyoid process; 4, muscular process; 5, petrosal crest; 6, occipital bone in formation of mastoid foramen; 3, apex; 
groove which concurs in formation of temporal canal; 4, opening of aqueductus vestibuli; 5, medial surface; 
7, groove for posterior meningeal artery. 6, petrosal crest; 7, internal acoustic meatus; 8, mus- 
cular process; 9, bulla ossea; 10, stylo-mastoid foramen; 
A.c., opening of aqueductus cochlee. 
tibuli), covered by a scale of bone. Below this there is a narrow fissure, the 
orifice of the aqueeductus cochlew (Apertura externa aqueductus cochlez). 
The anterior surface looks upward and forward. The greater part articulates 
with the parietal bone, but a small medial part faces into the cerebral fossa of the 
cranium. <A sharp border, the petrosal crest (Crista petrosa), separates this sur- 
face from the medial one. 
The posterior surface is slightly concave and is attached to the lateral part of 
the occipital bone. 
The base forms the lateral boundary of the foramen lacerum. It is very 
irregular and presents a number of important features. The hyoid process (Pro- 
cessus hyoideus) is a short rod which projects downward and forward below the 
base of the external acoustic process, inclosed in a bony tube; it is connected by a 
bar of cartilage with the hyoid bone. The stylo-mastoid foramen (F. stylo- 
mastoideum) is situated between the hyoid process and the mastoid process; it is 
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