"EOLOGY 





depressions for the arachnoid granulations, and at its back part, the openings of 

 the parietal foramina when these are present. It is crossed, in front, by the coronal 

 suture, and behind by the lambdoidal, while the sagittal lies in the medial plane 

 between the parietal bones. 



Upper Surface of the Base of the Skull (Fig. 193). The upper surface of the 

 base of the skull or floor of the cranial cavity presents three fossae, called the anterior, 

 middle, and posterior cranial fossae. 



Anterior Fossa (fossa cranii anterior').- The floor of the anterior fossa is formed 

 by the orbital plates of the frontal, the cribriform plate of the ethmoid, and the 

 small wings and front part of the body of the sphenoid ; it is limited behind by the 

 posterior borders of the small wings of the sphenoid and by the anterior margin 

 of the chiasmatic groove. It is traversed by the frontoethmoidal, sphenoethmoidal, 

 and sphenofrontal sutures. Its lateral portions roof in the orbital cavities and sup- 

 port the frontal lobes of the cerebrum; they are convex and marked by depressions 

 for the brain convolutions, and grooves for branches of the meningeal vessels. 

 The central portion corresponds with the roof of the nasal cavity, and is markedly 

 depressed on either side of the crista galli. It presents, in and near the median 

 line, from before backward, the commencement of the frontal crest for the attach- 

 ment of the falx cerebri; the foramen cecum, between the frontal bone and the crista 

 galli of the ethmoid, which usually transmits a small vein from the nasal cavity 

 to the superior sagittal sinus; behind the foramen cecum, the crista galli, the 

 free margin of which affords attachment to the falx cerebri; on either side of the 

 crista galli, the olfactory groove formed by the cribriform plate, which supports 

 the olfactory bulb and presents foramina for the transmission of the olfactory 

 nerves, and in front a slit-like opening for the nasociliary nerve. Lateral to either 

 olfactory groove are the internal openings of the anterior and posterior ethmoidal 

 foramina; the anterior, situated about the middle of the lateral margin of the olfac- 

 tory groove, transmits the anterior ethmoidal vessels and the nasociliary nerve; the 

 nerve runs in a groove along the lateral edge of the cribriform plate to the slit-like 

 opening above mentioned; the posterior ethmoidal foramen opens at the back part 

 of this margin under cover of the projecting lamina of the sphenoid, and transmits 

 the posterior ethmoidal vessels and nerve. Farther back in the middle line is the 

 ethmoidal spine, bounded behind by a slight elevation separating two shallow lon- 

 gitudinal grooves which support the olfactory lobes. Behind this is the anterior 

 margin of the chiasmatic groove, running lateralward on either side to the upper 

 margin of the optic foramen. 



The Middle Fossa (fossa cranii media) .The middle fossa, deeper than the pre- 

 ceding, is narrow in the middle, and wide at the sides of the skull. It is bounded 

 in front by the posterior margins of the small wings of the sphenoid, the anterior 

 clinoid processes, and the ridge forming the anterior margin of the chiasmatic 

 groove; behind, by the superior angles of the petrous portions of the temporals 

 and the dorsum sellae; laterally by the temporal squamae, sphenoidal angles of the 

 parietals, and great wings of the sphenoid. It is traversed by the squamosal, 

 sphenoparietal, sphenosquamosal, and sphenopetrosal sutures. 



The middle part of the fossa presents, in front, the chiasmatic groove and tuber- 

 culum sellae ; the chiasmatic groove ends on either side at the optic foramen, which 

 transmits the optic nerve and ophthalmic artery to the orbital cavity. Behind 

 the optic foramen the anterior clinoid process is directed backward and medialward 

 and gives attachment to the tentorium cerebelli. Behind the tuberculum sellae 

 is a deep depression, the sella turcica, containing the fossa hypophyseos, which lodges 

 the hypophysis, and presents on its anterior wall the middle clinoid processes. 

 The sella turcica is bounded posteriorly by a quadrilateral plate of bone, the dorsum 

 sellae, the upper angles of which are surmounted by the posterior clinoid processes : 

 these afford attachment to the tentorium cerebelli, and below each is a notch for 



