The infraorbital or superior dental canal extends almost horizontally from the maxillary foramen to the infraorbital foramen. It is placed at the upper edge of the inner plate of the maxilla, and traverses the maxillary sinus. Near the infraorbital foramen it gives off a small canal (Canalis alveolaris incisivus) which lies above the roots of the premolars and extends also into the premaxilla, carrying vessels and nerves to the teeth there.

Development.—The maxilla ossifies in membrane below and external to the cartilaginous nasal capsule. It has one chief center and a supplementary one in the region of the deciduous canine tooth (Martin).

The Premaxillae

The premaxillæ (Ossa incisiva) form the anterior part of the upper jaw and carry the incisor teeth. Each consists of a body and two processes, nasal and palatine.

The body is the thick anterior part which carries the incisor teeth. Its labial or superior surface is convex and smooth, and is related to the upper lip. The palatine or inferior surface is concave and presents a foramen a little behind its middle.^[15] The internal surface is rough, and joins the opposite bone; it is marked by a curved groove, which forms with that on the opposed surface, the incisive foramen (Foramen incisivum). The alveolar border (Limbus alveolaris) separates the palatine and labial surfaces; it is curved and thick, and presents three alveoli for the incisor teeth; behind the third alveolus it is rounded and free, forming part of the interalveolar space.

The nasal process (Processus nasalis) projects backward and upward from the body, forming here the lateral wall of the nasal cavity. The two surfaces, facial and nasal, are smooth and rounded. The superior border is free, thick, and smooth. The inferior border is dentated and joins the maxilla; at its anterior end it forms with the latter the alveolus for the permanent canine tooth.^[16] The posterior extremity fits into the interval between the nasal bone and the maxilla.

The palatine process (Processus palatinus) is a thin plate which forms the anterior part of the basis of the hard palate. Its nasal or superior surface has a longitudinal ridge which forms with that of the other side a groove for the septal cartilage. The palatine or inferior surface is flat. The internal border is serrated and meets its fellow at the median palatine suture. The external border is separated from the maxilla and the nasal process by the palatine cleft (Fissura palatina). The posterior extremity fits into the interval between the vomer and the palatine process of the maxilla.

Development.—The premaxilla ossifies from a single center. Fusion of the two bones is complete at the end of the third or the beginning of the fourth year.

The Palatine Bones

The palatine bones (Ossa palatina) are situated on either side of the posterior nares, and form the posterior margin of the hard palate. Each is twisted so as to form a horizontal and a perpendicular part.

The horizontal part (Pars horizontalis) is a narrow plate which forms the posterior part of the hard palate. It presents smooth nasal and palatine surfaces. The internal border meets its fellow at the median palatine suture, on the nasal aspect of which is the nasal crest. The anterior border joins the palate process of the maxilla at the transverse palatine suture, and forms with it the anterior palatine foramen. The posterior border is concave and free; it gives attachment to the aponeurosis of the soft palate.

The perpendicular part (Pars perpendicularis) is more extensive and forms the outer boundary of the posterior nares. The nasal or internal surface is in the greater part of its extent concave and smooth; it is marked by a narrow rough area to which the pterygoid bone is attached. Below this the bone curves outward, forming the pterygoid process. The maxillary or external surface (Facies maxillaris) presents three areas for consideration. The largest articulates with the maxilla; it is rough and is crossed by a groove which enters into the formation of the palatine canal. Behind this is a smooth part which assists in forming the pterygo-palatine fossa. The rough area below this is overlapped by the pterygoid process of the sphenoid bone. The superior border is perforated by the sphenopalatine foramen. Behind the foramen the two plates of the bone separate to inclose part of the sphenopalatine sinus. The inner plate curves inward to articulate with the vomer. The outer plate joins the maxilla and frontal and the orbital wing of the sphenoid.

Development.—The palatine bone ossifies in membrane from a single center.

The Pterygoid Bones

The pterygoid bones (Ossa pterygoidea) are narrow, thin, bent plates, situated on either side of the posterior nares. Each has two surfaces and two extremities. The internal surface is smooth, and forms part of the boundary of the posterior nares. The external surface articulates with the palatine, vomer, and sphenoid, concurring with the last in the formation of the pterygoid or Vidian canal. The anterior extremity is free, turned slightly outward, and forms the hamulus, a pulley around which the tendon of the tensor palati muscle is reflected.

Development.—The pterygoid ossifies in membrane from a single center.

The Nasal Bones

The nasal bones (Ossa nasalia) are situated in front of the frontal bones and form the greater part of the roof of the nasal cavity. They have an elongated triangular outline, wide behind, pointed in front. Each presents two surfaces, two borders, a base, and an apex.

The external or facial surface is smooth and is convex transversely; the profile contour is usually slightly wavy, with a depression about its middle and a variably prominent area in front.

The internal or nasal surface is smooth and concave from side to side. About in its middle it presents the superior turbinal crest (Crista conchalis dorsalis), which is parallel with the inner border, and has the superior turbinal bone attached to it. Most of this surface faces into the nasal cavity, but its posterior part enters into the formation of the frontal sinus; the latter area is marked off by an oblique ridge.

The internal border is straight, and meets the opposite bone at the nasal suture.

The external border is irregular. Its anterior third is free and concurs with the nasal process in forming the naso-maxillary notch (Incisura naso-maxillaris). Behind this it is scaly for articulation with the end of the nasal process, the maxilla, and the lacrimal, forming the naso-maxillary and naso-lacrimal sutures.

The base or posterior end is beveled and overlaps the frontal bone, forming the naso-frontal suture.

The apex is pointed and thin.

Development.—Each nasal bone ossifies in membrane from a single center. The nasal suture does not close completely even in old age. In some cases the two plates separate to inclose a small air-space in the posterior part.

The Lacrimal Bones

The lacrimal bones (Ossa lacrimalia) are situated at the anterior part of the orbit, and extend forward on the face to the posterior border of the maxilla. Each presents three surfaces and a circumference.

The external aspect is clearly divided into orbital and facial parts by the orbital margin. The orbital surface (Facies orbitalis) is triangular in outline, smooth and concave; it forms part of the inner and front wall of the orbit. Near the orbital margin it presents a funnel-like fossa for the lacrimal sac (Fossa sacci lacrimalis), which is the entrance to the osseous lacrimal canal. Behind this is a depression in which the inferior oblique muscle of the eye takes origin. The facial surface (Facies facialis) is more extensive, and has the form of an irregular pentagon. It is slightly convex and smooth in the foal, flattened in the adult. It usually bears the small lacrimal tubercle, situated nearly an inch (ca. 2 cm.) from the orbital margin.

The nasal or internal surface (Facies nasalis) faces into the frontal and maxillary sinuses. It is concave and very irregular, and is crossed almost horizontally by the osseous lacrimal canal (Canalis lacrimalis osseus).

The orbital margin (Margo orbitalis) is concave, rough above, smooth below.

The circumference articulates above with the frontal and nasal bones, below with the malar and maxilla, in front with the maxilla, and behind with the frontal. The various sutures so formed are designated by combinations of the names of the bones.

Development.—Each ossifies in membrane from a single center.

The malar or zygomatic bones (Ossa zygomatica) are placed between the lacrimal above and the maxilla below and in front. Each is irregularly triangular in outline and presents three surfaces, three borders, a base, and an apex.

The facial surface is smooth, slightly convex, wide in front, and narrow behind. At its lower part it presents the facial or zygomatic crest, which is continuous in front with the similar ridge on the maxilla and behind with the zygomatic process of the temporal; the crest is rough below, where the masseter muscle is attached to it.

The orbital surface is separated from the facial surface by the concave orbital margin. It is concave and smooth, and forms part of the lower and front wall of the orbit.

The nasal surface is concave and faces into the maxillary sinus. In the young foal a considerable part of it articulates with the maxilla.

The superior border articulates with the lacrimal chiefly, but to a small extent behind with the maxilla also.

The inferior border and the base articulate with the maxilla.

The apex is beveled above and is overlapped by the zygomatic process of the temporal bone.

Development.—Each ossifies in membrane from one or two centers.

The Turbinal Bones

These (Ossa turbinata) are delicate, scroll-like bones, four in number, which are attached to the lateral walls of the nasal cavity. They project into the cavity and greatly diminish its extent. Each is composed of a very thin lamina, cribriform in many places, and covered on both sides with mucous membrane in the fresh state. They are arranged in two pairs, superior and inferior.

The superior or naso-turbinal (Concha dorsalis)^[17] is somewhat cylindrical in form, small at its anterior part, and flattened transversely. It is attached to the turbinal crest of the nasal bone and the nasal plate of the frontal bone. The anterior part is rolled like a scroll one and a half times, thus inclosing a cavity which communicates with the middle meatus nasi. The arrangement is best seen on a cross-section (Fig. 37). The posterior part is not rolled, but its lower border is attached to the lateral nasal wall, thus helping to inclose a large space which is part of the frontal sinus. This cavity is separated from that of the scroll-like part by a transverse septum. The internal surface is flattened, and is separated from the septum nasi by a narrow interval, the common meatus (Meatus nasi communis). Another narrow passage, the superior meatus (Meatus nasi superior), separates the upper surface from the roof of the nasal cavity. The space between the lower surface and the inferior turbinal is the middle meatus (Meatus nasi medius). The anterior extremity is prolonged toward the nostril by two small bars of cartilage.

The inferior or maxillo-turbinal bone (Concha ventralis) is shorter and smaller posteriorly than the upper one. It is attached to the inferior turbinal crest, and consists, like the upper one, of an anterior coiled and a posterior uncoiled portion.

To express briefly the mode of coiling of the two bones of the same side we may say that they are rolled toward the septum and each other.

The inferior and posterior borders of the posterior part are attached to the maxilla, thus helping to inclose a cavity which is part of the maxillary sinus. The lower surface is separated from the floor of the nasal cavity by the inferior meatus (Meatus nasi inferior), which is much larger than the other nasal passages. The anterior extremity is prolonged to the nostril by a curved bar of cartilage.

Development.—Each ossifies in cartilage from a single center.

The vomer is a median unpaired bone, which assists in forming the lower part of the septum nasi. It is composed of a thin lamina which is bent (except in its posterior part) so as to form a narrow groove (Sulcus septi narium), in which the lower part of the perpendicular plate of the ethmoid bone and the septal cartilage are received. The lateral surfaces, right and left, are highest near the posterior end and diminish gradually to the anterior end; they are slightly convex dorso-ventrally, and are covered by the nasal mucous membrane during life. The inferior border is thin and free in its posterior third, and divides the posterior nares into right and left halves; in the remainder of its extent it is wider and is attached to the nasal crest. The anterior extremity lies above the ends of the palatine processes of the premaxillæ. The posterior extremity consists of two wings (Alæ vomeris) which extend outward below the body of the presphenoid; posteriorly they form a notch (Incisura vomeris), and laterally join the palate and pterygoid bones.

Development.—The vomer is primitively double, and ossifies from a center on either side in the membrane covering the cartilaginous septum nasi; the two laminæ then fuse below and form a groove.

The Mandible

The mandible or inferior maxilla (Mandibula) is the largest and the only movable bone of the face. The two halves of which it consists at birth unite during the second or third month, and it is usually described as a single bone. It carries the lower teeth, and articulates by its condyles with the squamous temporal on either side. It consists of a body and two rami.^[18]

The body (Corpus mandibulæ) is the thick anterior part which bears the incisor teeth. It presents two surfaces and a border. The lingual or superior surface (Facies lingualis) is smooth and slightly concave; during life it is covered by mucous membrane, and the tip of the tongue overlies it. The labial or mental surface (Facies mentalis) is convex and is related to the lower lip. It is marked by a median furrow which indicates the position of the primitive symphysis mandibulæ. The curved alveolar border (Limbus alveolaris) presents six alveoli for the incisor teeth, and a little further back two alveoli for the canine teeth in the male; in the mare the latter are usually absent or small.

The rami (Rami mandibulæ) extend backward from the body and diverge to inclose the submaxillary space (Spatium mandibulare). Each ramus is bent so as to consist of a horizontal part (Pars molaris) which bears the lower cheek teeth, and a vertical part (Ramus mandibulæ) which is expanded and furnishes attachment to powerful muscles; the term angle is applied to the most prominent part of the curve. The ramus presents two surfaces, two borders, and two extremities. The external surface is smooth and slightly convex from edge to edge on the horizontal part; at the junction with the body it presents the mental foramen (Foramen mentale), which is the external opening of the mandibular or inferior dental canal. On the vertical part it is somewhat concave and presents a number of rough lines for the attachment of the masseter muscle. The internal surface of the horizontal part is smooth, and presents a shallow longitudinal depression in its middle; above this there is often a faint mylo-hyoid line for the attachment of the muscle of like name. At the lower part of the junction with the body there is a small fossa for the attachment of the genio-hyoid and genio-glossus muscles. On the vertical part the surface is concave, and is marked in its lower and posterior part by rough lines for the attachment of the internal pterygoid muscle. In front of its middle is the mandibular or inferior maxillary foramen (Foramen mandibulare), which is the posterior orifice of the mandibular or inferior dental canal (Canalis mandibulæ). The canal curves downward and passes forward below the cheek teeth, opening externally at the mental foramen; it is continued into the body of the bone as a small canal (Canalis alveolaris incisivus), which carries the vessels and nerves to the incisor teeth. The superior or alveolar border forms anteriorly part of the interalveolar space; here it is thin. Behind this it is thick and is excavated by six alveoli for the lower cheek teeth. Behind the last alveolus it curves sharply upward and is narrow and rough. In the young foal there is commonly a small alveolus for the vestige of the first premolar (“wolf tooth”) close to the first large one. The inferior border of the horizontal part is nearly straight; it is thick and rounded in the young horse, becoming narrower and sharp in old subjects. At its posterior part there is a smooth impression (Incisura vasorum) where the facial vessels and parotid duct turn round the bone. Behind this point the border curves sharply upward, forming the angle (Angulus mandibulæ); this part is thick and has two roughened lips, internal and external, separated by a considerable intermediate space; near the condyle it becomes narrower. The anterior extremity joins the body. The superior extremity comprises the coronoid process in front and the condyle behind, the two being separated by the sigmoid notch (Incisura mandibulæ), through which the nerve to the masseter muscle passes. The coronoid process (Processus coronoideus) is thin transversely and curved slightly inward and backward. It projects upward in the temporal fossa, and furnishes insertion to the temporal muscle. The condyle (Capitulum mandibulæ) lies at a much lower level than the end of the coronoid process. It is elongated transversely and articulates with the squamous temporal through the medium of an articular disc. The part below the condyle is usually termed the neck (Collum mandibulæ); on its antero-internal part is a depression (Fovea pterygoidea) for the attachment of the external pterygoid muscle. The middle of the vertical part of the ramus consists to a large extent of a single plate of compact substance which may be so thin in places as to be translucent.

Development.—The mandible develops from two chief centers in the connective tissue which overlies the paired Meckel’s cartilages. At birth it consists of two symmetrical halves which meet at a median symphysis. Fusion usually occurs in the second or third month.

Age changes.—These are associated largely with the growth and later with the reduction of the teeth. In the young horse, in which the teeth are large and are in great part embedded in the bone, the body is thick and strongly curved, and the horizontal part of the ramus is also thick. Later, as the teeth are extruded from the bone, the body becomes flattened and narrower, and the horizontal part of the ramus is thinner, especially in its lower part; the angle and the impression in front of it are more pronounced.

The Hyoid Bone

The hyoid bone (Os hyoideum) is situated chiefly between the vertical parts of the rami of the mandible, but its upper part extends somewhat further back. It is attached to the petrous temporal bones by rods of cartilage, and supports the root of the tongue, the pharynx, and larynx. It consists of a body, a lingual process, and three pairs of cornua.

The body or basihyoid (Basis ossishyoidei) is a short transverse bar, compressed dorso-ventrally. The upper surface is concave and smooth in its middle, and presents at each end a convex facet or tubercle for articulation with the small cornu. The lower surface is slightly roughened for muscular attachment. The anterior border carries medially the lingual process. The posterior border is concave and smooth in its middle, and carries on either side the thyroid cornu. The body, the lingual process, and the thyroid processes are fused together, and may be compared to a spur or a fork with a very short handle.

The lingual process (Processus lingualis) projects forward medially from the body, and is embedded in the root of the tongue during life. It is compressed laterally and has a blunt-pointed free end. The lateral surfaces are slightly concave. The upper border is narrow, the lower thick and irregular.

The thyroid cornua or thyrohyoids (Cornua laryngea)^[19] extend backward and upward from the lateral parts of the body. They are compressed laterally (except at their junction with the body), and their posterior ends are connected with the anterior cornua of the thyroid cartilage of the larynx.

The small cornua or keratohyoids (Cornua minora) are short rods which are directed upward and forward from either end of the upper surface of the body. Each is somewhat constricted in its middle part and has slightly enlarged ends. The lower end has a small concave facet which articulates with the body. The upper 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 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.