The skull of the horse has the form of a long four-sided pyramid, the
base of which is posterior. It is convenient to exclude the mandible and
hyoid from present consideration. The division between the cranium
(Cranium cerebrale) and the face (Cranium viscerale) may be indicated
approximately by a transverse plane through the anterior margins of the
orbits.
The =superior= or =frontal surface= (Norma frontalis) is formed by the
upper part of the occipital, the interparietal, parietal, frontal, and
nasal bones. It may be divided into parietal, frontal, nasal, and
premaxillary regions. The parietal region extends from the occipital
crest to the parieto-frontal or coronal suture. It is marked medially by
the =external sagittal crest=, which bifurcates in front, the branches
becoming continuous with the =frontal crests=. The latter curve outward
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=.
[Illustration:
FIG. 33.—CRANIAL AND ORBITAL REGIONS OF SKULL OF HORSE, LATERAL VIEW.
THE ZYGOMATIC ARCH AND SUPRAORBITAL PROCESSES HAVE BEEN SAWN OFF.
1, Occipital condyle; 2, condyloid fossa; 3, paramastoid or styloid
process; 4, occipital crest; 5, external occipital protuberance; 6,
external auditory meatus; 7, mastoid process; 8, hyoid process; 9,
stylo-mastoid foramen; 10, muscular process; 11, foramen lacerum
anterius; 12, postglenoid process; 13, glenoid cavity; 14, temporal
condyle; 15, Vidian groove; 16, alar canal of pterygoid process
indicated by arrow; 17, temporal foramen; 18, ethmoidal foramen; 19,
optic foramen; 20, foramen lacerum orbitale; 21, maxillary foramen;
22, sphenopalatine foramen; 23, posterior palatine foramen; 24,
supraorbital foramen (opened); 25, lacrimal fossa; 26, depression
for origin of obliquus oculi inferior; 27, facial crest; 28,
maxillary tuberosity; 29, alveolar tuberosity; 30, hamulus of
pterygoid bone; _S.o._, supraoccipital; _P_, parietal; _S_, squamous
temporal; _B.o._, basioccipital; _B.s._, basisphenoid; _A.t._,
temporal wing of sphenoid; _A.o._, orbital wing of sphenoid;
_Pt.p._, pterygoid process of sphenoid; _P.p._, perpendicular part
of palate bone; _F,F′_, facial and orbital parts of frontal bone;
_L,L′_, orbital and facial parts of lacrimal bone; _M_, facial part
of malar bone; _M.x._, maxilla; _a_, parieto-occipital suture; _b_,
parieto-temporal or squamous suture; _c_, _d_, spheno-squamous
suture; _e_, palato-frontal suture; _f_, fronto-lacrimal suture.
]
The =lateral surface= (Norma lateralis) may be divided into cranial,
orbital, and maxillary or preorbital regions.
The =cranial region= presents the temporal fossa, the zygomatic arch,
and the outer part of the petrous temporal bone.
The =temporal fossa= is bounded internally by the sagittal and frontal
crests, externally by the temporal crest and the zygomatic arch, and
behind by the occipital 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 parieto-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 =condyle=
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 =external auditory process= projects outward through a deep notch in
the lower margin of the squamous temporal below the temporal crest. A
little further back is the =mastoid process=, crossed in its upper part
by a groove for the mastoid artery.
The =orbital region= comprises the orbit and the pterygo-palatine
=fossa=.
The =orbit= is a cavity which incloses the eyeball, with the muscles,
vessels, and nerves associated with it. It is not separated in the
skeleton from the temporal fossa. The =long axis= of the cavity, taken
from the optic foramen to the middle of the =inlet=, is directed
forward, outward, and slightly upward. The =inner wall= (Paries
medialis) is complete and extensive. It is concave and smooth, and is
formed by the frontal and lacrimal and the orbital wing of the sphenoid.
In its extreme anterior part is the fossa for the lacrimal sac. Behind
this is a small depression in which the inferior oblique muscle of the
eye arises; here the plate which separates the orbit from the maxillary
sinus is very thin. The =upper wall= (Paries superior) is formed by the
frontal and to a small extent by the lacrimal bone. It presents the
supraorbital foramen, which perforates the root of the supraorbital
process. The =lower wall= (Paries inferior) is very incomplete, and is
formed by the malar, the zygomatic process of the temporal, and to a
small extent by the maxilla. The =external boundary= (Paries lateralis)
is the supraorbital process. At the extreme posterior part is the
=orbital group of foramina=. Four are situated in front of the pterygoid
crest. Of these, the uppermost is the =ethmoidal= or =internal orbital
foramen=, which transmits the ethmoidal vessels and nerve. The =optic
foramen= is situated a little lower and further back; it transmits the
optic nerve. Immediately below the optic is the =foramen lacerum
orbitale=, which transmits the ophthalmic, third, sixth, and sometimes
the fourth nerve; commonly there is a very small =trochlear= or
=pathetic foramen= in the crest for the last-named nerve. The =foramen
rotundum= is below the foramen lacerum, from which it is separated by a
thin plate; it transmits the superior maxillary nerve. The =alar canal=
opens in common with the foramen rotundum, and the anterior opening of
the pterygoid or Vidian canal is also found here. The =temporal foramen=
(For. alare parvum) is just behind the pterygoid crest and on a level
with the foramen lacerum. It is the upper opening of a canal which leads
from the alar canal, and through it the anterior deep temporal artery
emerges. The =inlet= of the orbital cavity (Aditus orbitæ) is
circumscribed by a complete bony ring, which is nearly circular. Its
antero-inferior part (Margo infraorbitalis) is smooth and rounded; the
remainder (Margo supraorbitalis) is rough and irregularly notched.
During life the cavity is completed by the periorbita or ocular sheath,
a conical fibrous membrane, the apex of which is attached around the
optic foramen.
Below the orbital cavity is the =pterygo-palatine fossa=. Its wall is
formed by the pterygoid process, the perpendicular part of the palate
bone, and the tuber maxillare. Its deep anterior part (maxillary hiatus)
contains three foramina. The upper one, the =maxillary foramen=, is the
entrance to the infraorbital canal, which transmits the infraorbital
nerve and vessels. The =sphenopalatine foramen= transmits vessels and
nerves of like name to the nasal cavity. The lower foramen, the
=posterior palatine=, transmits the palatine artery and nerve to the
palatine canal. The upper part of the fossa is smooth, and is crossed by
the internal maxillary artery and the maxillary nerve. The lower part is
chiefly roughened for the attachment of the internal pterygoid muscle,
but is crossed in front by a smooth groove in which the palatine vein
lies.
The =maxillary= or =preorbital region= is formed chiefly by the maxilla,
but also by the premaxilla, and the facial parts of the lacrimal and
malar bones. Its contour is approximately triangular, the base being
posterior. It offers two principal features. The =facial crest= extends
forward from the lower margin of the orbit, and ends abruptly at a point
about an inch and a half (3 to 4 cm.) above the third or fourth cheek
tooth;[20] its inferior aspect is rough for the attachment of the
masseter muscle. The =infraorbital foramen= is situated in a transverse
plane about an inch (ca. 2 to 3 cm.) in front of the end of the crest
and about two inches (5 cm.) above it. The foramen opens forward, and
through it the infraorbital artery and nerve emerge. The surface over
the premolar teeth varies greatly with age, in conformity with the size
of the embedded parts of the teeth. In the young horse the surface here
is strongly convex, the outer plate of bone is thin and even defective
sometimes in places, and the form of the teeth is indicated by eminences
(Juga alveolaria). In the old animal the surface is concave on account
of the extrusion of the teeth from the bone. The downward curve of the
premaxilla is pronounced in the young subject, very slight in the aged.
The =inferior= or =basal surface= (Norma basalis), exclusive of the
mandible, consists of cranial, guttural, and palatine regions.
The =cranial region= (Basis cranii externa) extends forward to the vomer
and pterygoid processes (Fig. 28). At its posterior end is the =foramen
magnum=, flanked by the =occipital condyles=. External to the latter is
the =condyloid fossa=, in which is the =hypoglossal foramen=, which
transmits the hypoglossal nerve and the condyloid artery and vein.
Further outward are the =paramastoid= or =styloid processes= (Processus
jugulares) of the occipital bone. Extending forward centrally is a
prismatic bar, formed by the basilar part of the occipital and the body
of the sphenoid bone; at the junction of these parts are tubercles for
the attachment of the ventral straight muscles of the head. On either
side of the basilar part of the occipital is the =foramen lacerum basis
cranii=, bounded externally by the base of the petrous temporal bone. In
front of these the region becomes very wide on account of the lateral
extension of the =zygomatic processes=, bearing on the ventral aspect
the =condyle= and =glenoid cavity= for articulation with the mandible.
Beyond this the process turns forward and joins the =zygomatic process=
of the malar, completing the zygomatic arch and the surface for the
attachment of the masseter muscle. On either side of the body of the
sphenoid is the infratemporal fossa, formed by the temporal wing and the
root of the pterygoid process of the sphenoid bone. It is bounded in
front by the pterygoid crest, which separates it from the orbit and the
pterygo-palatine fossa. In it is the =pterygoid= or =alar foramen=,
which transmits the internal maxillary artery. A little lower is the
entrance to the pterygoid (Vidian) =canal=.
The =guttural region= presents the pharyngeal orifice of the nasal
cavity. This is elliptical and is divided in its depth medially by the
vomer into two =posterior nares= or =choanæ=. It is bounded in front and
laterally by the palate and pterygoid bones, behind by the vomer. It is
flanked by the hamular process of the pterygoid bones. The plane of the
opening is nearly horizontal, and the length is about twice the width.
The =palatine region= comprises a little more than half of the entire
length of the base of the skull (Fig. 31). The =hard palate= (Palatum
durum) is concave from side to side, and in its length also in the
anterior part. It is formed by the palatine processes of the premaxillæ
and maxillæ, and the horizontal parts of the palate bones. It is
circumscribed in front and laterally by the =superior alveolar arch=, in
which the upper teeth are implanted. The =interalveolar space= (Margo
interalveolaris) is that part of the arch in which alveoli are not
present. Behind the last alveolus is the =alveolar tuberosity=, and
internal to this is a =groove= for the palatine vein. In the middle line
is the =median palatine suture= (Sutura palatina mediana). In the line
of the suture, a little behind the central incisors, is the =foramen
incisivum=, through which the palato-labial artery passes. On either
side, parallel with the alveolar part of the maxilla, is the =palatine
groove= (Sulcus palatinus), which contains the palatine vessels and
nerve. It is continuous at the anterior palatine foramen with the
palatine canal, which is situated between the maxilla and the palate
bone. The =palatine cleft= (Fissura palatina) is the narrow interval
along the outer margin of the palatine process of the premaxilla; it is
closed in the fresh state by cartilage. Scattered along each side of the
palate are several vascular foramina. The =transverse palatine suture=
(Sutura palatina transversa) is about half an inch from the posterior
border. The latter is in a plane through the last molar teeth, and is
concave and free.
[Illustration: