Part 12

The =posterior extremity= is formed by the =xiphoid= or =ensiform cartilage= (Processus xiphoideus). This is a thin plate, connected in front with the last bony segment by a relatively thick, narrow neck, and expanding in nearly circular form behind and laterally. Its dorsal surface is concave, and gives attachment to the diaphragm. The ventral surface is convex. The free margin is very thin.

=Development.=—At birth the sternum of the horse consists of seven bony segments or sternebræ united by intersternebral cartilages. The last two sternebræ fuse in the second month, but the others do not usually unite completely even in old age. The sternebræ consist of very vascular spongy bone covered by a very thin layer of compact substance. The adult sternum thus consists to a very considerable extent of persisting cartilage, viz., the intersternebral cartilages, the ventral keel, and the extremities; in old age these undergo partial ossification.

THE THORAX

The bony thorax of the horse is remarkably compressed laterally in its anterior part, but widens greatly behind. The =anterior aperture= is oval and very narrow below; in a horse of medium size its greatest width is about 4 inches (10 cm.), and its height 7 to 8 inches (ca. 18 to 20 cm.). The =ventral wall= or =floor= is about 10 inches (40 cm.) long, and the =dorsal wall= or =roof= about 38 to 40 inches (95 to 100 cm.) long. The height from the last segment of the sternum to the seventh or eighth thoracic vertebra is about twice that of the anterior aperture; this is due to the obliquity and divergence of the roof and floor. The greatest width of the posterior aperture is about 20 to 24 inches (50 to 60 cm.). The intercostal spaces increase in width from the first to the seventh or eighth, and then diminish. Their average width is about 1¼ to 1½ inches (3 to 3.5 cm.).

THE BONES OF THE SKULL

(A) BONES OF THE CRANIUM

The bones of the cranium (Ossa cranii) are the Occipital, Sphenoid, Ethmoid, Interparietal, Parietal, Frontal, and Temporal. The first four are single, the others paired.

FIG. 26.—SKULL, ATLAS, AND AXIS OF HORSE, LATERAL VIEW.

8, Body of mandible; 28′, horizontal (molar) part of ramus; 30, vertical part of ramus; 9, zygomatic process of squamous temporal; 11, coronoid process of mandible; 12, supraorbital process; 13″, paramastoid (styloid) process of occipital; 19, orbit; 20, malar bone; 21, lacrimal bone; 22, nasal bone; 23, premaxilla; 23′, nasal process of premaxilla; 25, 29, canine teeth; 26, maxilla; 27, facial crest; 31, condyle of mandible; 32, atlas; 33, axis; x, wing of atlas; e, naso-maxillary notch. (After Ellenberger-Baum, Anat. für Künstler.) ]

THE OCCIPITAL BONE

The =occipital bone= (Os occipitale) is situated at the posterior part of the cranium, of which it forms the posterior wall and part of the ventral wall or base.

Its lower part is perforated centrally by a large, almost circular opening, the =foramen magnum= (Foramen occipitale magnum), at which the brain and spinal cord join. The foramen is bounded laterally and dorsally by the =lateral parts= of the bone, and ventrally by the =basilar part= or =process=. Above the lateral parts—but not entering into the formation of the foramen magnum—is the =squamous part=.

FIG. 27.—LINE DRAWING OF POSTERIOR HALF OF BASE OF SKULL OF HORSE, WITHOUT MANDIBLE. (Key to Fig. 28.)

1, Incisura carotica; 2, incisura ovalis; 3, incisura spinosa; 4, external orifice of parieto-temporal canal; 5, Eustachian canal; 6, petro-tympanic fissure; 7, external auditory canal; 8, hyoid process; 9, Vidian groove; 10, supraorbital process; A, basilar part of occipital; B, body of sphenoid; C, temporal wing of sphenoid; D, squamous temporal bone; E, petrous temporal bone; F, orbital part of frontal bone. ]

The =lateral parts= (Partes laterales) bear the =occipital condyles= (Condyli occipitales), which articulate with the atlas. The condyles are obliquely placed, wide apart dorsally, and separated by a small interval ventrally. The articular surface is curved so sharply in the dorso-ventral direction as to form a blunt ridge externally. The cranial surface is concave and smooth. External to the condyle is the =paramastoid= or =styloid process= (Processus jugularis), a strong flattened bar of bone which projects downward and backward; its external surface is convex and roughened for muscular attachment. Between the root of this process and the condyle is a smooth depression, the =condyloid fossa= (Fossa condyloidea inferior); in this is the =hypoglossal foramen= (Foramen hypoglossi), which transmits the nerve of like name.

The =basilar part= or =process= (Pars basilaris) is a strong, somewhat prismatic bar which extends forward from the ventral margin of the foramen magnum. It is much narrower in front than behind. The ventral surface is rounded. The cranial surface is concave and smooth; its posterior part, supports the medulla, and its anterior part has a shallow cavity on which the pons rests. The lateral borders are thin and sharp, and form the inner margin of the =foramen lacerum= (Foramen lacerum et jugulare). The anterior end has, in the young subject, a semicircular, flat, pitted surface which is attached to the body of the sphenoid bone by a layer of cartilage; in the adult there is complete fusion. On the ventral aspect of the junction are tubercles for the attachment of the ventral straight muscles of the head.

FIG. 28.—VENTRAL SURFACE OF SKULL OF HORSE, POSTERIOR HALF WITHOUT MANDIBLE.

The skull is inclined slightly. (Notation on key Fig. 27.) ]

The =squamous part= (Squama occipitalis) is the somewhat quadrilateral mass situated above the lateral portions, from which it remains distinct till the second year. The =outer surface= is crossed by a very prominent ridge, the =occipital crest=; the middle part of this is thick, transverse in direction, and forms the highest point of the skull when the head is in the ordinary position; laterally it becomes thinner and runs downward and forward to join the temporal crest. The crest divides the surface into two very unequal parts; the small anterior area (Planum parietale) presents a median ridge which is the posterior part of the external sagittal crest; the large area below the crest (Planum nuchale) also has a central eminence, the =external occipital protuberance=, on the sides of which the funicular part of the ligamentum nuchæ is attached. The =internal surface= is concave and presents a deep central depression and two shallower lateral ones which adapt it to the surface of the cerebellum.

The occipital bone is connected by suture with the interparietal, two parietals, and two temporals, and by synchondrosis with the sphenoid; the condyles articulate with the atlas.

=Development.=—The occipital ossifies in cartilage from =four centers=, and consists at birth of four pieces as described above. The lateral parts unite with the basilar at three to four months, and with the squama in the second year, when the bone is consolidated.

The parieto-occipital suture and the spheno-occipital synchondrosis are obliterated about the fifth year usually. The temporo-occipital suture partially ossifies in old subjects.

THE SPHENOID BONE

The =sphenoid bone= (Os sphenoidale) is situated in the base of the cranium, its central part lying in front of the basilar part of the occipital. It consists of a central part, the body, two pairs of wings, and two pterygoid processes.

The =body= (Corpus) is a cylindrical bar, flattened dorso-ventrally, and wider in front than behind. Its =ventral surface= (Facies externa) is convex in the transverse direction; and its anterior part is concealed to a large extent by the vomer and pterygoid bones. The =cerebral surface= (Facies cerebralis) presents the following features: (1) In front is a raised, flattened part which is subdivided by a median elevation into two slightly concave lateral areas; this part has a posterior, thin, free margin (Limbus sphenoidalis), which overlies the entrance to the optic foramina. (2) Just behind this and at a lower level is a smooth transverse depression, the =optic groove= (Sulcus chiasmatis), on which the optic chiasma rests. (3) From each end of this groove the =optic foramen= passes forward and outward to terminate in the posterior part of the orbital fossa. (4) Near the posterior end is a central depression, the =hypophyseal= or =pituitary fossa= (Fossa hypophysea), which lodges the hypophysis cerebri or pituitary body. On each side of this is an ill-defined groove for the internal carotid artery and the cavernous sinus. The =anterior end= is expanded, and is excavated to form the =sphenoidal sinuses.= These cavities extend back as far as the optic groove, and are usually continuous in front with the cavities in the vertical parts of the palate bones; they are separated by a complete septum which is not always median. The =posterior end= is flat and is joined to the basilar part of the occipital; at the line of junction there is dorsally a transverse elevation, the =spheno-occipital= crest (Crista sphenooccipitalis).

The =orbital wings= (Alæ orbitales) curve upward and somewhat outward from the sides of the body of the presphenoid. Their =inner= or =cerebral surfaces= are concave, and are marked by =digital impressions= (Impressiones digitatæ) for the gyri of the cerebrum. The =external surface= is convex and is largely concealed by the overlapping temporal wing and the squamous temporal and frontal bones; a narrow part of it (Facies orbitalis) is uncovered on the inner wail of the orbital cavity at the sphenoidal notch of the frontal bone. The =dorsal border= unites with the frontal bone at the spheno-frontal suture. The =anterior border= joins the ethmoid at the spheno-ethmoidal suture; at its lower part it concurs with the frontal in the formation of the =ethmoidal= (or internal orbital) =foramen=. The =posterior border= is overlapped by the temporal wing and the squamous temporal. The =root= of the wing is perforated by the =optic foramen=. Immediately below and behind the latter (i. e., beneath the root) is the =foramen lacerum orbitale= or =orbital fissure=. Below this, and separated from it usually by a thin plate, is a larger opening, the =foramen rotundum=, which is bounded externally by the root of the pterygoid process. Behind these foramina is the =pterygoid crest= (Crista pterygoidea), which is continued downward and forward on the pterygoid process; on its upper part may be found the small and inconstant =trochlear= (or pathetic) =foramen=. Just behind the crest is the =temporal foramen= (For. alare parvum), through which the anterior deep temporal artery emerges from the alar canal of the pterygoid process.

The =temporal wings= (Alæ temporales) extend outward and somewhat upward from the body of the postsphenoid; they are irregularly quadrilateral in outline. The =external surface= (Facies temporalis) enters into the formation of the infratemporal fossa, and bears the pterygoid process on its anterior part; at the junction with the body there is a small groove for the pterygoid nerve. The =internal surface= (Facies cerebralis) presents, at the junction with the body, two longitudinal =grooves= (Sulci nervorum). The outer groove is the larger, and leads forward to the foramen rotundum; it contains the maxillary nerve. The inner groove conducts to the orbital fissure, and contains the third, sixth, and ophthalmic nerves. The outer groove is bounded externally by a thin overhanging crest, on which is a small groove for the fourth nerve. The remainder of the surface is concave and supports the pyriform lobe of the brain. The =dorsal border= joins the squamous temporal at the spheno-squamous suture. The =anterior border= joins the orbital wing. The =posterior border= forms the anterior boundary of the foramen lacerum; it presents =three notches=, which are (from within outward) the carotid, oval, and spinous (Incisura carotica, ovalis, spinosa). The angle of junction of the dorsal and posterior borders articulates with the parietal bone.

The =pterygoid processes= (Processus pterygoidei) arise from the temporal wings and the body. They project downward and forward, and curve outward at the lower part. The root is perforated by the =alar canal= (Canalis alaris), which transmits the internal maxillary artery. From this canal a branch leads upward and forward to open at the temporal foramen. The =external surface= is concave, and is marked by lines for muscular attachment. The =internal surface= is largely concealed by the overlapping palate and pterygoid bones, with which it concurs in the formation of the =pterygoid= or =Vidian canal=.