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 cornua 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. ach is a thin plate, seven or eight inches (ca. 18 to 20 
em.) long, which is slightly curved in its length, so that the lateral surface is con- 
eave 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 cornua or epihyoids are small, wedge-shaped pieces or nodules 
interposed between the small and great cornua. They are usually transitory, 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 SKULL 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 surface (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 
