Part 13

The body (b) has the form of a flattened cylinder and has two surfaces and two borders. The lateral surface (Fig. 36) is smooth and presents near its cranial end a foramen (or sometimes two), the =mental foramen= (d), forming the cranial termination of the mandibular canal. At its caudal end is a deep fossa continuing on to the ramus, the =coronoid fossa=, or =masseteric fossa= (e).

FIG. 37.--MANDIBLE, MEDIAL SURFACE.

a, symphysis; b, body; c, ramus; d, mental foramina; e, coronoid fossa; f, mandibular foramen; g, angular process; h, coronoid process; i, condyloid process; 1, 2, 3, the three incisor teeth; 4, the canine tooth; 5, 6, the premolars; 7, the molar tooth.]

The medial surface (Fig. 37) is smooth and has near its caudal end a foramen, the =mandibular foramen= (f), which communicates with the mandibular canal leading lengthwise through the bone to the mental foramen. The cranial end is roughened for attachment to the bone of the opposite side.

The ventral border is smooth and rounded; it ends caudally in a blunt point, the =angular process= (g). The dorsal (alveolar) border is slightly curved and bears the sockets (=alveoli=) for the teeth. It is continuous with the cranial margin of the coronoid process.

The ramus is divided into two portions, the =coronoid= process (h) and the =condyloid= process (i). The coronoid process (h) extends dorsocaudad as a thin plate of bone with smooth surfaces and borders. Its outer surface is partly occupied by the =coronoid fossa= (e). The condyloid process (i) has the form of a semicylindrical transverse piece of bone attached to the caudal margin of the coronoid process. It articulates with the mandibular fossa of the temporal bone.

=Hyoid Bone.= =Os hyoideum= (Fig. 38 and Fig. 104).--The hyoid bone forms the support for the tongue and gives origin to muscles passing to the tongue and larynx. It also supports the thyroid cartilage (Fig. 104, 1).

a, body; b, c, d, e, cranial cornu; f, caudal cornu; b, ceratohyal; c, epihyal; d, stylohyal; e, tympanohyal; f, thyrohyal.]

It consists of a transverse bony bar, the =body= (Fig. 38, a) and of two =cornua= or horns attached to each end of the body.

The cranial cornu (lesser cornu of human anatomy) is the longer (Fig. 38, b-e). Each arises from the cranial face of the body at its lateral end, curves laterad, and then caudodorsad. It consists of four bony pieces movably united by cartilage.

The terminal piece is the =tympanohyal= (e); it is imbedded in the tympanic bulla just ventrad of the stylomastoid foramen. It is not therefore seen attached to the cornu after the latter has been separated from the skull. The other pieces become successively shorter toward the body, and are called =stylohyal= (d), =epihyal= (c), and =ceratohyal= (b).

The caudal cornua (f) (greater cornua of human anatomy) arise from the ends of the body. Each consists of a single piece of bone, the =thyrohyal= (f), which passes caudolaterad; its free end is united to a process of the thyroid cartilage (Fig. 104, 1).

=The Skull as a Whole.=--In the following description of the skull as a whole the mandible, hyoid, and ear-bones are not included.

The skull forms a bony box which contains the brain and is produced craniad into the =facial= portion which encloses the nasal cavity and forms the framework of the face.

In =dorsal view= (Fig. 39) the skull presents a smooth convex surface, broadest caudad, with the two =zygomatic arches= (g) curving out some distance laterally. The following bones are visible in dorsal view: the occipital (1), interparietal (2), parietals (3), temporals (4), frontals (5), malar or zygomatic bones (6), nasals (7), maxillaries (8), premaxillaries (9), and lachrymals (10).

1, occipital bone; 2, interparietal bone; 3, parietal bones; 4, temporal; 5, frontal; 6, malar; 7, nasal; 8, maxillary; 9, premaxillary; 10, lachrymal, a, lambdoidal ridge; b, external occipital tubercle; c, sagittal crest; d, parietal eminence; e, line which forms the dorsal boundary of the temporal fossa; f, zygomatic process of the frontal; g, zygomatic arch; h, frontal process of the malar; i, supraorbital arch; j, nares; k, foramen incisivum or anterior palatine foramen; l, sphenopalatine foramen; m, zygomatic process of the temporal; n, infraorbital foramen; o, opening of lachrymal duct.]

The caudal boundary of the dorsal surface is marked by the prominent =lambdoidal ridge= (a) which passes from the middle cranioventrad along each side to the root of the zygomatic arch: it is borne by the occipital and temporal bones. From the middle of the lambdoidal ridge a second ridge, the =sagittal crest= (c), passes craniad in the middle line across the interparietal bone: it varies greatly in extent, reaching in a very old and muscular cat to the cranial border of the parietals, while in kittens it does not exist. The most prominent portions of the skull in this region, just craniad of the middle of the parietal bones, are known as the parietal tubercles or eminences (d). A faint curved line (e) runs from the cranial end of the sagittal crest craniolaterad to the base of the zygomatic process of the frontal: it marks the dorsal boundary of the origin of the temporal muscle, and may therefore be considered the dorsal boundary of the temporal fossa. This fossa extends from its dorsal boundary as far laterad and caudad as the lambdoidal ridge (a), and as far craniad as a line connecting the tip of the zygomatic process of the frontal (f) with the frontal process of the malar (h). The temporal muscle takes origin from its surface.

The middle portion of the dorsal surface is formed by the frontals (5). Each frontal presents laterally a prominent =zygomatic process= (f), extending ventrolaterad toward a corresponding (frontal) process (h) of the malar bone. These two processes mark the boundary between the orbital fossa (craniad) and the temporal fossa (caudad). Craniad of the zygomatic process of the frontal a sharp margin separates the dorsal surface of the skull from the wall of the orbital fossa: this is the =supraorbital arch= or margin (i).

The cranial portion of the dorsal surface is formed by the maxillary (8), nasal (7), and premaxillary bones (9). Just craniad of the nasals, bounded ventrad and craniad by the premaxillaries, appears the large opening of the nares (j), leading into the nasal cavity.

The =zygomatic arch= (g) is formed by the zygomatic process of the temporal (m) and the malar or zygomatic bone (6). Each presents near its middle a prominent dorsocaudally directed process, the frontal process (h) of the malar bone. The zygomatic arch forms the lateral boundary of the temporal and orbital fossæ, which are separated by a line connecting the frontal process of the malar (h) and the zygomatic process of the frontal (f).

A portion of the floor of the orbit and the opening of the lachrymal canal (o) may also be seen in dorsal view; they are described in connection with the lateral surface.

The =caudal surface= of the skull is formed largely by the occipital bone (Fig. 17), surrounding the foramen magnum (Fig. 17, d). At the sides of the foramen magnum are the two prominent curved occipital condyles (e) for articulation with the atlas. Craniolaterad of the condyles, separated from them by a deep notch, are the jugular processes (f) of the occipital, closely applied to the caudal ends of the tympanic bullæ.

Dorsad of the foramen magnum are faint indications of a median ridge running dorsad, the =external occipital crest= (Fig. 17, i); this rises at its junction with the lambdoidal ridge to form the prominent =external occipital tubercle= (Fig. 39, b). The dorsal and dorsolateral boundaries of the posterior surface are formed by the lambdoidal ridge (Fig. 17, h; Fig. 39, a).

The =lateral surface= of the skull (Fig. 40) is much more complicated than the dorsal and posterior surfaces. Caudally the occipital condyles (a) and external occipital crest (b) are visible; dorsocaudad the sagittal crest (c).

Extending from the caudal end of the sagittal crest the lambdoidal ridge (d) is seen passing ventrocraniad to the tympanic bulla, thence craniad to the root of the zygomatic arch. In the ventral part of the caudal region the tympanic bulla (e) is visible with the jugular process (f) of the occipital pressed close against its caudal end. Just craniad of the jugular process the mastoid process (g) of the temporal rests against the side of the bulla. Beneath the cranial edge of this process is the opening of the stylomastoid foramen (h) for the seventh nerve, while just ventrad of the foramen is the small pit (i) in the tympanic bulla for the reception of the tympanohyal bone. Craniad of the stylomastoid foramen is the large opening of the external auditory meatus (j), leading into the middle ear.

Immediately dorsocraniad of the external auditory meatus the zygomatic arch begins as the zygomatic process (k) of the temporal bone. On the cranial surface of the base of this process is the deep mandibular fossa (l) for the condyle of the mandible. This fossa is bounded caudally by the prominent postmandibular process (m).

1, occipital bone; 2, interparietal; 3, parietal; 4, temporal; 5, 5′, frontal; 6, malar; 7, sphenoid; 8, palatine; 9, presphenoid; 10, maxillary; 11, nasal; 12, premaxillary; 13, incisor teeth; 14, canine; 15, 16, 17, premolars; 18, molar. a, occipital condyle; b, external occipital crest; c, sagittal crest; d, lambdoidal ridge; e, tympanic bulla; f, jugular process; g, mastoid process; h, stylo-mastoid foramen; i, pit for tympanohyal bone; j, external auditory meatus; k, zygomatic process of temporal bone; l, mandibular fossa; m, postmandibular process; n, zygomatic process of the frontal; o, supraorbital margin; p, external pterygoid fossa; q, sphenopalatine foramen; r, orbital fissure; s, internal pterygoid fossa; t, hamulus; u, foramen ovale; v, foramen rotundum; w, optic foramen; x, opening of lachrymal canal; y, infraorbital foramen.]

All that portion of the lateral surface of the skull which lies craniodorsad of the lambdoidal ridge may be divided (excluding the zygomatic arch) into three main parts, the =temporal fossa=, the =orbital fossa=, and the =face=. The boundaries of the temporal fossa have been given. The orbital fossa is bounded externally by a prominent semicircular ridge formed chiefly by the zygomatic arch, the zygomatic process of the frontal (n), and the supraorbital arch (o) of the frontal, which may be traced to the cranial root of the zygomatic arch. The orbital fossa may be considered to end caudally and ventrally at the level of the optic foramen (w); ventrad of it are certain smaller fossæ. Immediately ventrad is the long =external pterygoid fossa= (p), from which arises part of the external pterygoid muscle. This fossa begins at the sphenopalatine foramen (q) and extends caudad to the orbital fissure (r); it is separated by a ridge from the orbital fossa. Caudoventrad of the external pterygoid fossa and separated from it by a sharp ridge is the small narrow =internal pterygoid= fossa (s), which extends ventrad without interruption on to the surface of the hamulus (t) and caudad to within two or three millimeters of the tympanic bulla. From it the internal pterygoid muscle takes origin. The hamulus (t) projects caudoventrad in this region, forming a prominent feature in a lateral view.