THE BONES OF THE HEAD 151 



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

 small wings, jugum sphenoidale, and ethmoidal spine of the sphenoid. 

 It is limited posteriorly by the posterior or Sylvian border of the 

 small wing of the sphenoid at either side, and by the limbus sphen- 

 oidalis in the centre. It is subdivided into a central and two lateral 

 parts. 



The central portion, which is depressed, is formed by the cribri- 

 form plate of the ethmoid and the ethmoidal spine and jugum 

 of the sphenoid. In the middle line it presents the crista galli, 

 which gives attachment to the falx cerebri. In front of this is the 

 foramen caecum, which, when pervious, transmits an emissary- vein 

 passing between the intracranial superior longitudinal sinus and the 

 veins of the roof of the nose. At each side of the crista gaUi are 

 the nasal slit for the nasal nerve and anterior ethmoidal artery ; 

 the cribriform foramina for the filaments of the olfactory bulb ; 

 the cranial opening of the anterior ethmoidal canal for the anterior 

 ethmoidal artery and nasal nerve ; and the cranial opening of the 

 posterior ethmoidal canal for the posterior ethmoidal artery and 

 spheno-ethmoidal branch of the nasal nerve. The last two open- 

 ings are situated at the outer side of the cribriform plate, external 

 to the olfactory groove which marks it. Directly beneath each 

 half of the cribriform plate is the corresponding nasal fossa. 



Each lateral portion of the anterior fossa is irregularly convex, and 

 forms the roof of the orbit. It is formed by the orbital plate of the 

 frontal and the small wing of the sphenoid. It is very thin, and, 

 except over the small wing of the sphenoid, it presents digitate 

 impressions for the convolutions of the orbital surface of the frontal 

 lobe of the cerebrum, which it supports. The sutures in the anterior 

 fossa cire the fronto-ethmoidal, fr on to- sphenoidal, and spheno- 

 ethmoidal. 



Middle Fossa. — This fossa is on a lower level thm the anterior. 

 It is bounded in front by the posterior or Sylvian border of the small 

 wing of the sphenoid at either side, and by the limbus sphenoidalis 

 in the centre. Behind, it is limited by the superior border of the 

 petrous portion of the temporal at either side, and by the dorsum 

 sellae of the sphenoid in the centre. It presents a central and two 

 lateral divisions. The central division is formed by the superior sur- 

 face of the body of the sphenoid, posterior to the limbus sphenoidalis. 

 Each lateral division, which is much depressed, is formed anteriorly 

 by the superior surface of the great wing of the sphenoid, externally 

 by part of the squamous portion of the temporal, and posteriorly 

 by the superior surface of the petrous portion of that bone. It lodges 

 the temporo-sphenoidal lobe of the cerebrum, and it presents the 

 following sutiu-es : the spheno-parietal ; squamous or squamo- 

 parietal ; squamo-sphenoidal ; and petro-sphenoidal. 



The central division presents the following parts: the optic 

 groove and olivary eminence for the optic commissure ; the optic 

 foramen of each side for the optic nerve and ophthalmic artery; 

 the anterior clinoid process of each side ; the sella turcica or 



