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).

[Fig. 40].—Skull, Side View.

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.

Four foramina leading into the cranial cavity are visible in a lateral view of the skull, craniad of the tympanic bulla. The one nearest the bulla is the foramen ovale (u) for the third division of the fifth nerve; next craniad of this is the foramen rotundum (v) for the second division of the fifth nerve. These two foramina pierce the alisphenoid: just craniad of them, between the alisphenoid and the orbitosphenoid, is the large orbital fissure (r) (foramen lacerum anterius), which transmits the third, fourth, and sixth cranial nerves and the first division of the fifth. Dorsocraniad of the orbital fissure is the optic foramen (w), for the optic nerve.