176 ANATOMY OF THE RABBIT 



extending backward to the foramen magnum and containing, in 

 the natural condition, the cerebellum and related posterior portions 

 of the brain. It is partly set off from the middle cranial fossa 

 by a fold of the dura mater, the tentorium cerebelli, which projects 

 inward from the dorsal and lateral walls of the skull. This fold 

 is usually found adhering to the internal surface of the skull, 

 unless the latter has been very thoroughly cleared, and in all 

 cases its position is indicated by a low ridge of bone. The marked 

 difference in diameter between the middle and posterior cranial 

 fossae is accounted for by the great thickness of the auditory por- 

 tion of the skull. The anterior surface of the periotic bone will be 

 observed to form an extensive posterior wall for the middle cranial 

 fossa. 



The floor of the middle and posterior cranial fossae is not 

 smooth, like the external base of the skull, but presents in its 

 anterior portion a prominent elevation, the sella turcica, which is 

 borne on the body of the posterior sphenoid. The sella turcica 

 contains a large central depression, the hypophyseal fossa (fossa 

 hypophyseos), which in the natural condition lodges the hypophysis 

 or pituitary body. The aperture of the fossa is partly enclosed 

 laterally by a pair of pointed posterior clinoid processes (processus 

 clinoidei posteriores), the tips of which are directed forward; and 

 a corresponding pair of anterior clinoid processes lie at the anterior 

 end of the fossa, with the tips directed backward. The posterior, 

 and also dorsal wall of the fossa, described as the dorsum sellae, 

 leads by an abrupt curve backward on to the floor of the posterior 

 cranial fossa, the sloping portion of the floor, or clivus, supporting 

 in the natural condition the pons and medulla oblongata. Toward 

 the anterior end of the middle cranial fossa, the lateral walls of the 

 skull are greatly compressed, so that the anterior portion of the 

 basicranium, especially the body of the anterior sphenoid, is largely 

 excluded from the cranial cavity. The usually paired optic fora- 

 mina are here confluent, there being a single aperture for the 

 transmission of the optic nerves. The posterior ventral boundary 

 of this aperture contains a broad groove, the sulcus chiasmatis, 

 which in the intact animal lodges the optic chiasma. 



In the anterior cranial fossa the floor is largely formed by a 

 perforated area, borne on the cribriform plate (lamina cribrosa) 



