Part 7

The =zygomatic= or =temporal process= (Processus temporalis) projects backward above and external to the tuberosity; it is overlapped by the corresponding part of the malar and also articulates with the zygomatic process of the temporal. A small curved plate extends inward from it and joins the frontal and palate bones, forming part of the floor of the orbit.

The =palatine process= (Processus palatinus) is a plate which projects horizontally inward from the lower part of the inner surface of the body. It forms the greater part of the basis of the hard palate. Its =superior= or =nasal surface= is smooth and concave transversely; on its anterior part, close to the inner border, is a shallow groove in which the organ of Jacobson is situated. The inferior or =palatine surface= is slightly concave from side to side, and presents along its outer part the =palatine groove= (Sulcus palatinus). The groove is a continuation of the palatine canal, and contains the palatine artery and nerve. The =internal border= unites with its fellow to form the median palatine suture; its nasal aspect bears the nasal crest, which forms, with that of the opposite process, a groove for the vomer. The =posterior border= unites with the horizontal part of the palate bone at the transverse palatine suture.

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 BONES

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

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.