[Fig. 42].—Skull, with Dorsal Surface Removed, showing the Cranial and Nasal Cavities.

a, foramen magnum; b, caudal end of hypoglossal canal; c, jugular foramen; d, internal auditory meatus; e, tentorium, forming the cranial boundary of the cerebellar fossa; f, dorsum sellæ; g, sella turcica; h, anterior clinoid processes; i, foramen ovale; j, foramen rotundum; k, orbital fissure; l, optic foramen; m, chiasmatic groove; n, presphenoid bone; o, cribriform plate; p, lamina perpendicularis of ethmoid; q, labyrinths of ethmoid; r, nares; s, foramina incisiva or anterior palatine foramina; t, infraorbital foramen; u, opening of the lachrymal canal; v, caudal opening of posterior palatine canal; w, sphenopalatine foramen; x, frontal process of the malar; y, zygomatic process of the temporal; z, appendicular fossa, in the petrous bone.

Cavities of the Skull

([Figs. 42] and [43]).—The bones of the cranial portion of the skull enclose the cranial cavity for the brain; the facial bones enclose the nasal cavity, for the olfactory organ.

The cranial cavity is divisible into three principal fossæ: the cerebellar fossa ([Fig. 43], I) caudad, for the cerebellum; the cerebral fossa (II) in the middle, for the cerebrum; the small olfactory fossa (III) at the cranial end for the olfactory bulb of the brain.

The cerebellar fossa (I) is bounded caudally by the occipital bone enclosing the foramen magnum ([Fig. 42], a). Its ventral surface is formed by the basilar portion of the occipital and the petrous portions of the temporals; its lateral surface by the mastoid portions of the temporals and parts of the parietals and occipital. Its roof is formed by the parietals and interparietal. Craniad the cerebellar fossa is partly separated from the cerebral fossa by the tentorium ([Fig. 42], e; [Fig. 43], f) formed by the two parietals: this encloses a quadrangular opening by which the two fossæ communicate. The caudal, dorsal, and lateral walls of the cerebellar fossa are deeply marked by fossæ for the lobes of the cerebellum; the small appendicular fossa ([Fig. 43], e), forming a deep indentation in the petrous bone near its dorsocaudal end, is particularly noticeable.

The following openings are found in the walls of the cerebellar fossa. Caudad is the large foramen magnum ([Fig. 42], a) by which the brain-cavity communicates with the vertebral canal. Near the caudal margin of the foramen magnum, on its lateral side, just mediad of the dorsal end of the occipital condyle, is the caudal opening of the condyloid canal ([Fig. 43], a) which passes craniad through the substance of the occipital bone to open just caudad of the petrous: it transmits a vein. The condyloid canal varies greatly in size in different specimens. A few millimeters craniad of the edge of the foramen magnum on the floor of the fossa is the small opening of the hypoglossal canal ([Figs. 42] and [43], b), for the twelfth nerve. Just craniad of this, at the caudomedial border of the petrous, is the large jugular foramen (c). On the petrous itself, near the middle, is the internal, auditory meatus (d) divided into the dorsal facial canal for the seventh nerve, and a ventral passage for the eighth nerve. At the cranial end of the cerebellar fossa is the large opening bounded by the free edges of the tentorium.

The cerebral fossa forms much the largest part of the cranial cavity. It is bounded by the parietals ([Fig. 43], 3′), squamous portions of the temporals (4), frontals (8), the sphenoid (5), and presphenoid (6). A slight rounded ridge on its lateral wall at about the position of the suture between the frontals and parietals separates a smaller cranial portion sometimes called the anterior fossa, from a larger caudal portion sometimes known as the middle fossa of the cranial cavity. The walls of the cerebral cavity are marked with numerous ridges and shallow furrows for the cerebral convolutions.

The floor of the cerebral cavity is bounded caudad by the prominent dorsum sellæ ([Fig. 42], f; [Fig. 43], g), just craniad of which is the rounded depression known as the sella turcica ([Fig. 42], g; [Fig. 43], h), for lodgment of the hypophysis. A number of foramina pierce the floor of the cavity in this region. Just ventrad of the cranial tip of the petrous portion of the temporal is the small foramen lacerum (medius). Craniad and laterad of this is a row of four foramina: the caudal one is the foramen ovale ([Fig. 42], i); then come in order the foramen rotundum (j), the orbital fissure (k), and the optic foramen ([Fig. 42], l; [Fig. 43], k). The two optic foramina are connected by the shallow transverse chiasmatic groove ([Fig. 42], m), for the optic chiasma. Another small foramen continues caudad from a groove on the floor of the orbital fissure; this opens on the ventral surface of the sphenoid, between the wing and the body of the bone. The groove and foramen constitute the pterygoid canal, which transmits a nerve,—the nerve of the pterygoid canal, or Vidian nerve.