THE ARCHITECTURE OF THE CRANIUM. 



221 



many openings. The whole of the anterior fossa is very thin ; so is the sella turcica, 

 being just over the sphenoidal sinus. A chain of openings crosses the middle fossa 

 on either side. The temporal bone is practically crossed by the external and internal 

 meatuses and the middle ear, besides containing other cavities. Thus the petrous 

 is brittle, although the bone is very dense. A rim of comparatively firm bone 

 extends around two-thirds of the skull, starting on each side from the occipital 

 protuberance, which may be even two centimetres in thickness, along the line of the 

 lateral sinus to the supramastoid ridge ; it follows the line of origin of the zygoma, 



Fig. 254. 



Ethmoidal spine 



Crista sralH 



Foramen caecum 



Frontal sinus 



Cribriform plate 



Optic foramen 



Olivary emi- 

 nence 



Ant. clinoid 

 process 



Post, clinoid 

 process 



Lingula 



Middle lacer- 

 ated foramen 



Hiatus 

 Fallopii 



Int. auditory -- 

 meatus 



Post, lacer- 

 ated or jugu- 

 lar foramen 



Ant. condy- 

 loid foramen 



Sella turcica 



Sphenoidal 

 fissure (con- 

 cealed) 



Foramen ro- 

 tundum 



Foramen 

 ovale 



Foramen spi- 

 nosum 



Inf. petrosal 

 sinus 



Basilar 

 groove 



Sup. petrosal 



Lateral sinus 



Post, condy- 

 loid foramen 



Lateral sinus 



Torcular Herophili 



Base of skull from above. 



and ends in the infratemporal crest on the great wing of the sphenoid. A 

 median ridge strengthens the skull in both the frontal and occipital regions. 

 The average thickness of the vault is about four millimetres. It is thick through- 

 out the frontal region and at the parietal eminences, a thin area lying behind and 

 below the latter. The Pacchionian depressions may almost perforate the skull. It 

 is very thin in the squamous part of the temporal ; less so in the superior occipital 

 fossae. 



If the base of a skull be held to the light and examined from within, the 



