THE BASE OF THE SKULL. 105 



along the surface of the ethmoid to the slit-like opening above mentioned ; while 

 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. Farther back in the middle line is the ethmoidal spine, 

 bounded behind by a slight elevation, separating two shallow longitudinal grooves 

 which support the olfactory lobes. Behind this is a transverse sharp ridge, run- 

 ning outward on either side to the anterior margin of the optic foramen, and 

 separating the anterior from the middle fossa of the base of the skull. The 

 anterior fossa presents, laterally, depressions for the convolutions of the brain 

 and grooves for the lodgement of the anterior meningeal arteries. 



The Middle Fossa, deeper than the preceding, is narrow in the middle line, 

 but becomes wider at the side of the skull. It is bounded in front by the poste- 

 rior margin of the lesser wing of the sphenoid, the anterior clinoid process, and 

 the ridge forming the anterior margin of the optic groove ; behind, by the superior 

 border of the petrous portion of the temporal and the dorsum ephippi ; externally 

 by the squamous portion of the temporal, anterior inferior angle of the parietal 

 bone, and greater wing of the sphenoid. It is traversed by four sutures, the 

 squamo-parietal, spheno-parietal, squamo-sphenoidal, and petro-sphenoidal. 



In the middle line, from before backward, is the optic groove, behind which 

 lies the optic commissure ; the groove terminates on each side in the optic fora- 

 men, for the passage of the optic nerve and ophthalmic artery ; behind the optic 

 groove is the olivary process, and laterally the anterior clinoid processes, to Avhich 

 are attached processes of the tentorium cerebelli. Farther back is the sella 

 turcica, a deep depression which lodges the pituitary gland, bounded in front 

 by a small eminence on either side, the middle clinoid process, and behind 

 by a broad square plate of bone, the dorsum ephippi, surmounted at each 

 superior angle by a tubercle, the posterior clinoid process ; beneath the latter 

 process is a notch, for the sixth nerve. On each side of the sella turcica is the 

 cavernous groove : it is broad, shallow, and curved somewhat like the Italic 

 letter/; it commences behind at the foramen lacerum medium, and terminates 

 on the inner side of the anterior clinoid process, and presents along its outer mar- 

 gin a ridge of bone. This groove lodges the cavernous sinus, the internal carotid 

 artery, and the nerves of the orbit. The sides of the middle fossa are of con- 

 siderable depth ; they present depressions for the convolutions of the brain and 

 grooves for the branches of the middle meningeal artery ; the latter commence 

 on the outer side of the foramen spinosum, and consist of two large branches, an 

 anterior and a posterior ; the former passing upward and forward to the anterior 

 inferior angle of the parietal bone, the latter passing upward , and backward. 

 The following foramina may also be seen from before backward : Most anteriorly 

 is the foramen lacerum anterius, or sphenoidal fissure, formed above by the lesser 

 wing of the sphenoid ; below, by the greater wing ; internally, by the body of the 

 sphenoid ; and sometimes completed externally by the orbital plate of the frontal 

 bone. It transmits the third, the fourth, the three branches of the ophthalmic 

 division of the fifth, the sixth nerve, some filaments from the cavernous plexus of 

 the sympathetic, the orbital branch of the middle meningeal artery, a recurrent 

 branch from the lachrymal artery to the dura mater, and the ophthalmic vein. 

 Behind the inner extremity of the sphenoidal fissure is the foramen rotundum, for 

 the passage of the second division of the fifth or superior maxillary nerve ; still 

 more posteriorly is seen a small orifice, the foramen Vesalii, an opening situated 

 between the foramen rotundum and ovale, a little internal to both : it varies in size 

 in different individuals, and is often absent ; when present, it transmits a small vein. 

 It opens below into the pterygoid fossa, just at the outer side of the scaphoid 

 depression. Behind and external to the latter opening is the foramen ovale, which 

 transmits the third division of the fifth or inferior maxillary nerve, the small 

 meningeal artery, and the small petrosal nerve. 1 On the outer side of the foramen 

 ovale is the foramen spinosum, for the passage of the middle meningeal artery ; and 



1 See footnote, p. 74. 



