[Illustration: 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.