Part 17

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) 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.

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) 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.

FIG. 32.—HYOID BONE OF HORSE, VIEWED FROM THE SIDE AND SOMEWHAT FROM IN FRONT.

a, Body; b, lingual process; c, thyroid cornu; c′, cartilage of c; d, small cornu; e, middle cornu; f, great cornu; f′, muscular angle of great cornu; g, cartilage of great cornu. (Ellenberger-Baum, Anat. d. Haustiere.) ]

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.