80 



Anatomy of the Rabbit. 



6.m. 



small division lodgins^ in the natural condition the olfactory bulbs of 

 the brain. The middle cranial fossa, the largest division of the cavity, 

 lodges the enlarged cerebral hemispheres. The posterior cranial fossa is a 

 small division extending backward to the foramen magnum, and lodging 

 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 posi- 

 tion is indicated by a low 

 ridge of bone. The marked 

 difference in diameter between 

 the middle and posterior cra- 

 nial fossae is accounted for by 

 the great thickness of the 

 auditory portion 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 pro- 

 minent elevation, the sella 

 turcica, which is borne on the 

 body of the posterior sphe- 

 noid. It contains a large 

 central depression, the hypo- 

 physial fossa (fossa hypophy- 

 seos), which in the natural 

 condition lodges the hypo- 

 physis 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 



Fig. 32. Ventral surface of the skull: AS, alisphenoid 

 (ala magna); B, basioccipital (basilar portion of oc- 

 cipital); BS, basisphenoid (body of posterior sphenoid); 

 EXO, exoccipital; M, maxilla; PL, palatine; PMX, 

 premaxilla; PR, presphenoid (body of anterior sphe- 

 noid); SO, supraoccipital (squamous portion of occi- 

 pital); SQ, squamosal; T, tympanic; ZY, zygomatic. 



ch, choana; c.hy., hypoglossal canal; c.o., occipital 

 condyle; f.c.e., external carotid foramen; f.in., incisive 

 foramen; f.j, jugular foramen; f.L, foramen lacerum; 

 f.m., mandibular fossa; f.m.c, foramen magnum; 

 f.p.m., greater palatine foramen; f.s.a., anterior sphe- 

 noidal foramen; m.a.e., osseous portion of external 

 acoustic meatus; p.i., jugular process; p.o.e., external 

 occipital protuberance; p. pi., palatine process of rnaxilla; 

 p.pm., palatine process of premaxilla; p.pt., medial and 

 lateral laminae of pterygoid process of posterior sphenoid; 

 s.m., spina masseterica. 



