Part 8

The =great cornua= or =stylohyoids= are much the largest parts of the bone. They are directed upward 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 slightly curved in its length, so that the external surface is concave and smooth. The internal surface is convex and smooth. The borders are thin. The upper extremity is large and forms two angles; the upper angle is connected by a rod of cartilage with the hyoid process of the petrous temporal bone; the lower angle is somewhat thickened and rough for muscular attachment. The lower 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. The latter has primarily two centers (Martin).

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;[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.

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.

THE CRANIAL CAVITY

This cavity incloses the brain, with its membranes and vessels. It is relatively small and is ovoid in shape.

The =superior wall= or =roof= (Calvaria) is formed by the supraoccipital, interparietal, parietal, and frontal bones. In the middle line is the internal sagittal crest, which joins the crista galli in front, and furnishes attachment to the falx cerebri. Posteriorly the crest is continued by the sharp anterior margin of the =tentorium osseum=, which projects downward and forward into the cavity, and gives attachment to the tentorium cerebelli by its sharp lateral edges. Behind this the roof is grooved centrally for the middle lobe or vermis of the cerebellum. Transverse grooves pass from the base of the tentorium osseum to the parieto-temporal canals. The anterior part of the roof is hollowed by the frontal sinus. The occipital part is very thick and strong.

The =lateral walls= are formed by the occipital, parietal, temporal, and frontal bones, and in part by the orbital wings of the sphenoid. Each is crossed obliquely by the =petrosal crest=, which concurs with the projecting margin of the parietal bone and the tentorium osseum in dividing the cavity into cerebral and cerebellar compartments. Behind the crest is a depression for the lateral lobe (hemisphere) of the cerebellum. Below this are the internal auditory meatus and the openings of the aquæductus vestibuli and aquæductus cochleæ.

The roof and lateral walls are marked by digital impressions and vascular grooves.

The =inferior wall= or =floor= (Basis cranii interna) may be regarded as forming three fossæ. The =anterior fossa= (Fossa cranii anterior) supports the frontal and olfactory parts of the cerebrum. It is formed chiefly by the presphenoid, and lies at a higher level than the middle fossa. In front the fossa is divided medially by the =crista galli=, lateral to which are the deep =ethmoidal= or =olfactory fossæ= for the olfactory lobes. The =ethmoidal= or =internal orbital foramen= perforates the cranial wall at the outer side of these fossæ. Further back the central part of the surface is slightly elevated, and is flanked by shallow depressions which support the olfactory peduncles. Posteriorly is a bony shelf which covers the entrance to the =optic foramina=; the edge of this shelf and the posterior borders of the orbital wings of the sphenoid may be taken as the line of demarcation between the anterior and middle fossæ. The =middle fossa= (Fossa cranii media) is the widest part of the cavity. It extends backward to the internal spheno-occipital and petrosal crests, thus corresponding to the postsphenoid. In its middle is a small fossa, the sella turcica, in which the pituitary body, or hypophysis cerebri, is situated. On either side are two grooves for nerves; the inner one transmits the ophthalmic, third, and sixth nerves to the foramen lacerum orbitale; the outer one leads to the foramen rotundum, and lodges the maxillary nerve. External to the grooves is a depression for the pyriform lobe of the cerebrum. The =posterior fossa= (Fossa cranii posterior) corresponds to the basilar part of the occipital bone. It contains the medulla, pons, and cerebellum. In front is a median depression (Fossa pontis) for the pons. The surface behind this is concave transversely and slopes gently downward to the foramen magnum; it supports the medulla. On either side are the =foramen lacerum basis cranii= and the =hypoglossal foramen=.

The =anterior= or =nasal wall= is formed by the cribriform plate of the ethmoid, which separates the cranium from the nasal cavity. It is perforated by numerous foramina for the passage of the olfactory nerve-bundles.

THE NASAL CAVITY

The =nasal cavity= (Cavum nasi) is a longitudinal passage which extends through the upper part of the face. It is divided into right and left halves by a median =septum nasi=. The =lateral walls= are formed by the maxilla, premaxilla, and the perpendicular part of the palate bones. Attached to them are the turbinal bones, which subdivide each nasal fossa into three =meatuses= (Meatus nasi). This wall is crossed obliquely by the =canal= and =groove= for the =naso-lacrimal duct=, and its posterior part is perforated by the =sphenopalatine foramen=. The =superior wall= or =roof= is formed by the frontal and nasal bones. It is concave from side to side, and nearly straight longitudinally, except in the posterior part, where it curves downward. It presents a median elevation, the =nasal crest=. The =inferior wall= or =floor= is formed by the palatine processes of the premaxillæ and maxillæ, and the horizontal parts of the palate bones. It is wider but considerably shorter than the roof. It is concave transversely, and nearly horizontal from before backward, except in the posterior third, where there is a slight declivity. The anterior part presents a median groove for the cartilaginous septum, and a furrow for the organ of Jacobson on either side. On either side of the palatine processes of the premaxillæ is the =palatine cleft=.

The =septum nasi= is incomplete in the macerated skull. It is formed by the perpendicular plate of the ethmoid behind, and the vomer below. In the fresh state it is completed by a plate of cartilage.