IXTERIOR OF THE SKULL 83 



The ethmoidal spine of the splienoid. 



Furrows for meningeal arteries. 



The MIDDLE CRANIAL FOSSA presciits a central isthmus and two lateral depressed 

 portions. It is limited anteriorly by the posterior borders of the lesser wings of the 

 si)henoid and the anterior margin of the optic groove. The posterior limits are the 

 dorsum ephippii and the superior borders of the petrosals. Laterally it is bounded 

 by the squamosals, tlie great wings of the sphenoid, and the parietal bones. The 

 lloor is formed by the body and greater wings of the sphenoid, and the anterior 

 surfaces of tlie petrosals. It contains the following sutures : spheno-parietal, 

 petro-sphenoid, squamo-sphenoidal, s(}uamous, and a portion of the transverse 

 suture. The central portion or isthmus of the middle fossa presents the following 

 points from before l)ackwards: — 



The optic groove, which lodges the optic chiasma. 



The optic foramina, transmitting the optic nerve and ophthalmic artery. 



The olivary process, indicating the line of ankylosis betAveen pre- and post- 

 sphenoid. 



The anterior clinoid processes. 



The pituitary fossa, with the middle clinoid processes, and grooves for the 

 internal carotid arteries. The dorsum ephippii, with the posterior clinoid 

 processes, and notches for the sixth pair of cranial nerves. 



This central de})ression is in direct relation with the parts of the brain sur- 

 rounding the circle of Willis. 



The lateral depressions receive the temporo-sphenoidal lobes of the brain, and 

 are marked by numerous furrows roughly corresponding to the convolutions of the 

 cerebrum. Numerous narroAV diverging channels pass upwards from the fossa 

 towards the vertex ; these lodge the ramifications of the middle and small meningeal 

 arteries. 



The f<3llowing openings occur on each side of this fossa: — 



The sphenoidal fissure, leading into the orbit and transmitting the third, 

 fourth, ophthalmic division of the fifth and the sixth nerves, and ophthalmic vein. 



In the greater Aving of the sphenoid near its union with the frontal l;)one there 

 are small openings allowing twigs of the middle meningeal artery to enter the 

 orbit. 



The foramen rotundum, which conducts tlie second division of the fifth nerve 

 into the sphcno-niaxillary fossa. 



The foramen ovale : this transmits the third division of the fifth nerve with its 

 motor root, the small meningeal artery, and the small superficial petrosal nerve. 



The foramen Vesalii (not always present) for a small vein. 



The foramen spinosum, for the middle meningeal artery and its venae 

 comites. 



The middle lacerated foramen, which transmits at its inner angle the internal 

 carotid artery, with the carotid plexus of nerves. 



On the posterior wall of this fossa the objects of interest are: — 



A depression which lodges the Gasserian ganglion. 



The hiatus Fallopii, for the great superficial petrosal nerve, and a twig from 

 the middle meningeal artery. 



A foramen for the small superficial petrosal nerve. 



A minute foramen for the external superficial petrosal nerve. 



An eminence formed by the superior semicircular canal. 



Anterior and slightly external to the ridge formed by the superior semicircular 

 canal, the bone is exceedingly thin and translucent. This is the roof of the tym- 

 panum (tegmen tympani). When the dura mater is in situ, the depression lodging 

 the Gasserian ganglion is converted into a foramen, traversed by the fifth nerve. 

 The notch in the side of the dorsum ephippii for the sixth ners'e is also a foramen 

 when the dura mater is present. In many skulls the middle clinoid process is pro- 

 longed to meet the anterior clinoid process, and thus forms a foramen for the inter- 

 nal carotid artery. The grooves for the middle meningeal arteries are sometimes 

 (•anals or tunnels in a part of their course, especially in old skulls. The grooves 

 radiate from the foramen spinosum and extend to the vault. The bones most 

 deei)ly marked are the squamosal, the greater wing of the sphenoid, and the 

 parietal. 



