56 THE SKELETON OF THE HORSE 



The external surface (Facies temporalis) is convex, and enters into the forma- 

 tion of the temporal fossa. From its lower part there springs the zygomatic 

 process (Processus zygomaticns), which forms the external l^oundary of the tem- 

 poral fossa. It is at first directed outward, and is wide and flattened dorso-ven- 

 trally. It then turns forward, becomes narrower, and is twisted so that its surfaces 

 are internal and external. Its anterior end is pointed and joins the zygomatic 

 process of the malar bone, with which it forms the zygomatic arch (Arcus zygo- 

 maticus). The narrow anterior part has a convex outer surface and a concave 

 inner one. Its upper border has a rough area for articulation with the supraorbital 

 process of the frontal. Its lower border is wide and rough. The wide posterior 

 part presents on its ventral face a surface for articulation with the condyle of the 

 mandible. This surface consists of a transversely elongated condyle (Tuberculum 

 articulare), behind which is the glenoid fossa (Fossa mandibularis). The fossa is 

 limited behind by the post-glenoid process, the anterior surface of which is articu- 

 lar. Behind this process is a fossa in which is the external opening of the parieto- 

 temporal canal. The dorsal surface is concave and forms the outer boundary of 

 the temporal fossa. The superior border is sinuous and is continuous behind with 

 the temporal crest. 



The posterior process (Processus posterior) springs from the posterior part of 

 the squama. Its external surface is crossed by the temporal crest, which forms 

 here the outer limit of the temporal fossa. The internal surface forms the outer 

 boundary of the parieto-temporal canal, and is elsewhere applied to the petrous 

 portion. It divides into two branches, upper and lower; the upper branch unites 

 with the occipital bone, while the lower one curves downward behind the external 

 auditory process and overlaps the mastoid process. 



The superior border of the squamous temporal articulates with the parietal, 

 forming the parieto-temporal suture. The inferior border joins the temporal wing 

 of the sphenoid at the spheno-squamous suture. The anterior border unites with 

 the frontal l)one, and the posterior with the parietal. 



2. The petrous temporal (Os petrosum) is placed between the occipital behind 

 and the parietal in front, and is largely overlapped externally by the squamous 

 temporal. It has the form of a four-sided pyramid, the base of which is ventral. 



The external surface is mainly concealed by the squamous temporal, but two 

 features are visible. A short tube of bone, the external auditory process, protrudes 

 from the lowest part through the notch of the squamous temporal. The process is 

 directed outward, upward, and a little forward. It gives attachment to the annular 

 cartilage of the ear. Its lumen, the external auditory meatus (Meatus acusticus 

 externus), conducts to the cavity of the middle ear (tympanum) in the dry skull, 

 but is separated from it by the tympanic membrane in the natural state. The 

 mastoid process projects ventrally in the interval between the posterior process 

 of the squamous temporal and the root of the paramastoid (or styloid) process of 

 the occipital bone; its outer surface is crossed by a curved groove which leads to 

 the parieto-temporal canal. 



The internal surface faces into the cerebellar fossa of the cranium. It is con- 

 cave and smooth but irregular. In its lower part is the entrance to a short canal, 

 the internal auditory meatus, which transmits the seventh and eighth cranial nerves. 



The fundus of the meatus is divided by a crest into a superior and an inferior fossa. In the 

 superior one is the origin of the facial canal, which curves through the bone and opens exter- 

 nally at the stylo-mastoid foramen; it transmits the facial (seventh cranial) nerve. The in- 

 ferior fossa presents small foramina for the passage of fibers of the auditory (eighth cranial) 

 nerve. 



Behind the meatus and near the posterior margin of the surface is the slit-like 

 opening of the aquaeductus vestibuli, covered by a scale of bone. Below this is a 

 narrow fissure, the orifice of the aquaeductus cochleae. 



