Part 6
The =base forms= the outer boundary of the foramen lacerum basis cranii. It is very irregular and presents a number of important features. The =hyoid process= is a short rod which projects downward and forward below the base of the external auditory process, inclosed in a bony tube; it is connected by a bar of cartilage with the hyoid bone. The =stylo-mastoid foramen= is situated between the root of the hyoid process and the mastoid process; it is the external opening of the facial canal, through which the facial nerve emerges. The =bulla ossea= is a considerable eminence situated centrally; it is thin-walled and incloses a cavity which is part of the tympanum. The =muscular process=[14] is a sharp spine which projects downward and forward from the anterior part of the base; it gives origin to the tensor and levator palati muscles. External to the root of the preceding is the small =petro-tympanic= or =Glaserian fissure= (Fissura petro-tympanica) for the passage of the chorda tympani nerve. The osseous =Eustachian tube= is a semicanal at the inner side of the root of the muscular process; it leads to the tympanum. At the inner side of the preceding is the slit-like orifice of the petrosal canal, which communicates with the facial canal.
The =apex= projects upward and backward between the squamous temporal and the occipital bone.
=Development.=—The petrous temporal may be regarded as consisting of petro-mastoid and tympanic parts. The latter includes the external auditory process, the bulla ossea, and the muscular process; it is developed in membrane. The petro-mastoid is developed in the cartilaginous ear capsule. Its petrous part consists of very dense bone which contains the labyrinth or internal ear and forms the inner wall of the tympanum.
The =parieto-temporal canal= (Meatus temporalis) is a continuation of the transverse groove which extends outward from the base of the tentorium osseum. It is directed downward, forward, and somewhat outward, and opens externally in front of the root of the auditory process. It is bounded by the squamous temporal externally, the petrous behind, and the parietal in front and internally. Several foramina open from it into the temporal fossa. It contains a large vein (Vena cerebralis dorsalis), the continuation of the transverse sinus of the dura mater.
The =foramen lacerum basis cranii= (Foramen lacerum et jugulare) is a large irregular opening in the cranial base, bounded internally by the basilar part of the occipital bone, externally by the petrous temporal, and in front by the temporal wing of the sphenoid. It consists of a large anterior part (Foramen lacerum anterius), and a narrow posterior part (Foramen lacerum posterius s. jugulare). It transmits the internal carotid artery, the middle meningeal artery, the mandibular, ninth, tenth, and eleventh cranial nerves, and the inferior cerebral vein.
In the fresh state the foramen is occupied by a dense fibrous membrane which is perforated by apertures for the various structures transmitted. Thus there are three openings in front for the internal carotid artery, the mandibular nerve, and the middle meningeal artery; these are (from within outward) the foramen caroticum, ovale, spinosum.
(B) BONES OF THE FACE
The bones of the face (Ossa faciei) are the Maxilla, Premaxilla, Palatine, Pterygoid, Nasal, Lacrimal, Malar, Superior Turbinal, Inferior Turbinal, Vomer, Mandible, and Hyoid. The last three are single, the others paired.
THE MAXILLAE
The =maxillæ= are the principal bones of the upper jaw and carry the upper cheek teeth. They are situated on the lateral aspect of the face, and articulate with almost all of the facial bones and the frontal and temporal also. For description each may be divided into a body and two processes.
The =body= (Corpus maxillæ) presents two surfaces, two borders, and two extremities. The =external= or =facial surface= (Facies lateralis) is somewhat concave in front and convex behind. On its posterior part is a horizontal ridge, the =facial= or =zygomatic crest= (Crista facialis); in a skull of medium size its anterior end is about an inch and a half (3 to 4 cm.) above the third or fourth cheek tooth, and it is continued behind by a corresponding ridge on the malar bone. About two inches (5 cm.) above and a little in front of the anterior end of the crest is the =infraorbital foramen= (Foramen infraorbitale); this is the external opening of the infraorbital canal.
In the young horse the anterior part of the surface is convex over the embedded parts of the teeth. As the latter are extruded the surface flattens and becomes concave in old subjects.
The =internal= or =nasal surface= (Facies nasalis) is concave dorso-ventrally; it forms the greater part of the lateral wall of the nasal cavity. Its upper part is crossed obliquely forward and downward by the shallow =lacrimal groove= (Sulcus lacrimalis), which contains the naso-lacrimal duct; in the adult the posterior part of the groove is converted into a canal, which is continuous with that on the inner surface of the lacrimal bone. Below the groove is the =inferior turbinal crest= (Crista conchalis inferior), to which the inferior turbinal bone is attached. Lower down and parallel with the turbinal crest is the =palatine process=, which projects inward like a shelf. Behind this the surface is rough for articulation with the palate bone; this area is crossed by a groove which concurs with a furrow on the palate bone in the formation of the =palatine canal=. The posterior part of the bone is excavated to form part of the maxillary sinus.
The =superior border= is irregular and scaly. Its anterior part is grooved and its posterior part beveled for articulation with the nasal process of the premaxilla and the nasal and lacrimal bones.
The =inferior= or =alveolar border= (Processus alveolaris) is thick, and presents six large cavities, the =alveoli=, for the cheek teeth. The alveoli are separated by transverse =interalveolar septa=. There is often a small alveolus for the first premolar (“wolf tooth”) close to the first large one. Further forward the border is narrow and forms part of the =interdental= or =interalveolar space= (Margo interalveolaris). Behind the last alveolus is a rough area, the =alveolar tuberosity=.
The =anterior extremity= is pointed. It joins the premaxilla, and forms with it the alveolus for the canine tooth.
The =posterior extremity= forms a rounded prominence, the =maxillary tuberosity= (Tuber maxillare). Internal to the tuberosity is a deep cavity, the maxillary hiatus, in which are three foramina. The upper one, the =maxillary foramen=, leads into the infraorbital canal. The lower one, the =posterior palatine foramen=, is the entrance to the palatine canal. The =sphenopalatine foramen= perforates the inner wall of the hiatus and opens into the nasal cavity.
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.
[Illustration:
FIG. 31.—UPPER JAW OF HORSE ABOUT FOUR AND A HALF YEARS OLD, VENTRAL VIEW.
1, 1, Posterior nares; 2, vomer; 3, horizontal part of palate bone; 4, anterior palatine foramen; 5, palatine groove; 6, transverse palatine suture; 7, median palatine suture; 8, palate process of maxilla; 9, palate process of premaxilla; 10, foramen incisivum; 11, malar bone; 12, maxilla; 13, anterior end of facial crest; 14, interalveolar space; I. 1–3, incisor teeth; _C_, canine tooth; _P1_, first premolar or “wolf” tooth. ]
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.