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 fossae. 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 aquseductus vestibuli (Apertura externa aquaeductus ves- 



%^'lM 



Fig. 



40. — Left Petrous Temporal 

 Lateral View. 



Bone or Horse; 



1, External acoustic meatua; 2, mastoid process; 3, 

 hyoid process; 4, muscular process; 5, petrosal crest; 6, 

 groove which concurs in formation of temporal canal; 

 7, groove for posterior meningeal artery. 



Fig. 4L — Left Petrous Temporal Bone of Horse; 

 Postero-medial View. 

 1, Mastoid process; 2, notch which concurs with 

 occipital bone in formation of mastoid foramen; 3, apex; 

 4, opening of aquaeductus vestibuli; 5, medial surface; 

 6, petrosal crest; 7, internal acoustic meatus; 8, mus- 

 cular process; 9, bullaossea; 10. stylo-mastoid foramen; 

 A.c, opening of aquaeductus cochleae. 



tibuli), covered by a scale of bone. Below this there is a narrow fissure, the 

 orifice of the aqueeductus cochleae (Apertura externa aquaeductus cochleae). 



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 



