The Skull as a Whole

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.

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; 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. ). 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. ). 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.

The posterior or nuchal surface (Norma occipitalis) is formed by the occipital bone. It is trapezoidal in outline, wider below than above, concave dorso-ventrally, convex transversely. It is separated from the superior surface by the occipital crest, and from the lateral surfaces by the superior curved lines (Lineæ nuchæ superiores). Below the crest are two rough areas for the attachment of the complexus muscles. A little lower is a central eminence on the sides of which the ligamentum nuchæ is attached. At the lowest part centrally is the foramen magnum, at which the brain and spinal cord meet; this is bounded laterally by the occipital condyles, which are flanked by the paramastoid or styloid processes (Processus jugulares).

The apex of the skull is formed by the bodies of the premaxillæ and mandible, carrying the incisor teeth.