THE SKULL AS A WHOLE 73 



a small concave facet which articulates with the body. The dorsal end articulates 

 with the great cornu, or with the middle cornu when present. 



The great comua or stylohyoids are much the largest parts of the bone. They 

 are directed dorsally and backward, and are connected above with the base of the 

 petrous temporal bones. Each is a thin plate, seven or eight inches (ca. 18 to 20 

 cm.) long, which is shghtly curved in its length, so that the lateral surface is con- 

 cave and the medial surface is convex; both are smooth. The borders are thin. 

 The dorsal extremity is large and forms two angles; the articular angle is connected 

 by a rod of cartilage with the hyoid process of the petrous temporal bone; the 

 muscular angle is somewhat thickened and rough for muscular attachment. The 

 ventral extremity is small, and articulates with the small or the middle cornu. 



The middle comua or epihyoids are small, wedge-shaped pieces or nodules 

 interposed between the small and great cornua. They are usually transitorj^, and 

 unite with the great cornua in the adult. 



Development. — The hyoid ossifies in the cartilages of the second and third 

 visceral arches. Each part has a separate center, except the lingual process, which 

 ossifies by extension from the body. In the foal there is a separate nucleus at each 

 end of the body which intervenes between the latter and the thyroid cornu; it 

 articulates with the small cornu. The anterior part of the lingual process may be 

 a separate piece. 



THE SKUIT, AS A "WHOLE 



The skull of the horse has, as a whole, the form of a long, four-sided pyramid, 

 the base of which is posterior. It is convenient, however, to exclude the mandible 

 and hyoid from present consideration. The division between the cranium (Cra- 

 nium cerebrale) and the face (Cranium viscerale) may be indicated approxi- 

 mately by a transverse plane through the anterior margins of the orbits. 



The dorsal or frontal sxtrface (Norma frontalis) is formed by the squamous 

 part of the occipital, interparietal, parietal, frontal, nasal, and premaxil- 

 lary bones. It may be divided into parietal, frontal, nasal, and premaxillary 

 regions. The parietal region extends from the nuchal crest to the parieto-frontal 

 suture. It is marked medially by the external parietal crest, which bifurcates in 

 front, the branches becoming continuous with the frontal crests. The latter curve 

 outward on either side to the root of the supraorbital process. The frontal region 

 is the widest part of the surface, and is smooth and almost flat. It is bounded in 

 front by the naso-frontal suture. On either side of it is the root of the supraorbital 

 process, pierced by the supraorbital foramen. The nasal region is convex from 

 side to side, wide behind, narrow in front. Its profile is in some cases nearly 

 straight; in others it is undulating, with a variably marked depression about its 

 middle and at the anterior end. The premaxillary region presents the osseous 

 nasal aperture (Apertura nasalis ossea) and the foramen incisivum. 



The lateral surface (Norma lateralis) (Fig. 28) may be divided into cranial, 

 orbital, and preorbital or maxillary regions. 



The cranial region presents the temporal fossa, the zygomatic arch, and the 

 outer part of the petrous temporal bone. The temporal fossa is bound medially 

 by the parietal and frontal crests, laterally by the temporal crest and the zygomatic 

 arch, and behind by the nuchal crest. Its upper and middle parts are rough for 

 the attachment of the temporal muscle. In its lower posterior part are several 

 foramina which communicate with the temporal canal. The fossa is continuous 

 in front with the orbital cavity. The zygomatic arch is formed by the zygomatic 

 processes of the temporal, malar, and maxilla. Its ventral face presents the con- 

 dyle and glenoid cavity for articulation with the lower jaw, through the medium 

 of the articular disc. Behind the glenoid cavity is the postglenoid process. The 



