THE RABBIT. 271 



upwards and outwards, firmly ankylosed, even in young 

 rabbits, one to either side of the basisphenoid, and pro- 

 duced below on each side into two laminae, which unite 

 with one another along their anterior edges, forming a 

 dihedral angle open behind. These form the pterygoid 

 process of the alisphenoid : with the inner lamina the 

 pterygoid bone ( 68) articulates. 



50. The sphenoidal fissure, a large notch on each side 

 between the basisphenoid and alisphenoid ; it transmits 

 the third, fourth, and sixth nerves, as well as the first and 

 second divisions of the fifth. 



51. The presphenoid, a laterally compressed bone, 

 joined behind to the basisphenoid by synchondrosis (carti- 

 laginous union), so that in the dry skull there is always a 

 considerable interval between the two bones. It forms the 

 inferior and anterior boundary of a large oval aperture 

 which puts the two orbits in communication with one 

 another, and both in communication with the brain- 

 case : this is the optic foramen, and transmits the 

 optic nerves ( 485). On its upper or cranial surface the 

 presphenoid is produced behind into the two anterior 

 clinoid processes : in front of the optic foramen it gives 

 off two irregular outwardly diverging laminae, which form 

 part of the inner wall of the orbit, articulating above 

 with the inferior border of the orbital process of the frontal. 



52. The orbitosphenoids : two wing-like laminse 

 directed outwards and slightly backwards, projecting one 

 from each side of, and completely ankylosed with, the pre 

 sphenoid : they form the superior and posterior boundary 

 of the optic foramen. 



53. The squamosal : a bone of very irregular form, 

 articulating above with the parietal and frontal, in front 

 with the orbital process of the frontal and the orbito- 



