INTEEXAL EASE OF THE SKULL. 



63 



in-egular digitate impressions corresponding to the convolutions of the 

 brain. The thinnest portions of the cranial wall are the cribriform 

 plate of the ethmoid and the orbital plates of the frontal bone, in both 

 of which the diploe is entirely absent ; the bone is also tliin and 

 compact in the middle part of the inferior occipital fossae, and in the 

 squama and glenoid fossa of the temporal. 



The upper part of the cranial cavity consists of a single vaulted 

 dome formed by the frontal, parietal, and occipital bones. It is marked 

 by the superior longitudinal grooves, by shallow cerebral impressions, 

 by small ramified meningeal grooves, and by Pacchionian fosste of 

 varying depth. The lower part or internal basis of the skull is divided 

 into three fossoe, named anterior, middle, and posterior. 



The anterior fossa, formed by the orbital plates of the frontal bone, 

 the small wing of the sphenoid, and the cribriform plate of the ethmoid, 

 supports the anterior lobes of the brain. It sinks into a hollow at the 



Fig. 53. — LxTERNAL Fig. 53. 



Basis op the Skull. 

 (A, T.) i 



1, anterior fossa and 

 roof of the orbit, as 

 formed by the frontal 

 bone, marked by im- 

 jDressions of cerebral 

 convolutions ; between 

 2 and 3, the foramen 

 Cfficum, crista galli and 

 cribriform plate of eth- 

 moid ; 3, ethmoidal 

 spine of the sphenoid ; 

 4, lesser wing of sphe- 

 noid terminating poste- 

 riorly in the anterior 

 clinoid process, inside 

 which is the optic fora- 

 men ; 5, placed in the 

 pituitary fossa, behind 

 the olivarj' eminence and 

 transverse gi'oove of the 

 optic commissure ; 6, 

 dorsum sell«, termina- 

 ting in the posterior 

 clinoid processes, and 

 to the side of these the 

 groove of the internal 

 carotid artery ; 7, fora- 

 men rotundum, in front 

 of which, but not seen in 

 the iigure, is the foramen 

 lacerum orbitale ; 8, foramen ovale ; 9, foramen spinosum ; 10, on the ridge of the petrous 

 bone, near its apex, and to the inside of the hollow occupied by the Gasserian ganglion ; 

 in front of this is the foramen lacerum medium ; 11, in front of the eminence of the 

 superior semicircular canal of the labyrinth, and behind the hiatus Fallopii ; 12, the pro- 

 minent ridge of the petrous bone, marked by the superior j)etrosal gi'oove ; 13, the 

 posterior surface of the petrous bone — to the inside, the meatus auditorius internus, 

 behind, the scale of bone covering the aqueduct of the vestibule ; 14, basilar gi'ouve ; 

 15, anterior condylar foramen ; 16, jugular part of the foramen lacerum posteriiLs ; 17, 

 groove of the lateral sinus ; 18, internal occipital protuberance, and running down from 

 it the internal occipital spine ; between 17 and 18, the upper part of the groove of the 

 lateral sinus, between 17 and 16, the lower part ; 19, cerebellar fossa. 



