CLASSIPICATION, AKD PHYLOGENT OP THE DINOENITHID^. 391 



e. The cerebral or prosencephalic fossse for the cerebral hemispheres. 

 1^. The olfactory or rhinencephalic fossas for the olfactory lobes. 



The metencephalic fossa is furnished by the post-pituitary part of the basis cranii ; it 

 is gently concave both from before backwards and from side to side, and is terminated 

 in front by the dorsum sellse, while behind it is continued on to the flattened dorsal 

 surface of the occipital condyle. Near its posterior end is seen on each side the large 

 vagus foramen [va.for.) and mesiocaudad of this the two small condyloid foramina 

 {con. for.). 



In front of the vagus foramen and separated from it by a vertical bar of bone about 

 2 mm. wide is the internal auditory meatus (fig. 16, int.aud.vi.), a shallow pit 

 containing the foramina for the facial and auditory nerves. The minute abducent 

 foramen for the sixth nerve lies on a line joining the internal auditory meatus with 

 the middle of the dorsum sellse (fig. 15, abdfor.), and about 5 or 6 mm. from the 

 latter. Between the abducent foramina a median aperture of considerable size is 

 sometimes found, in one instance communicating with the pituitary fossa ; this is 

 the internal opening of the posterior basicranial fontanelle. 



The cerebellar fossa (fig. 11, cer.fos.) is in the roof of the cranium and is bounded 

 mainly by the supraoccipital and parietals above and by the pro-otics laterally. It is 

 bounded behind by the dorsal edge of the occipital foramen, and in front by the 

 median portion of the tentorial ridge, while ventrally it passes insensibly on each side 

 into the metencephalic fossa. It is marked by a series of transverse grooves corres- 

 ponding with the gyri of the cerebellum. On each side, about 3 or 4 mm. above the 

 internal auditory meatus, is the smzll floccular fossa, which varies considerably in size 

 in different individuals. 



The mesencephalic fossa (mesenfos.) lies in the alisphenoid, laterad of the dorsum 

 sellee (fig. 10) and is bounded externally by the ventral portion of the tentorial ridge. 

 It is largely occupied by a shallow depression for the root of the trigeminal nerve, 

 and from this depression the trigeminal foramen proceeds directly outwards (fig. 16, 

 trig for.), and the tunnel-like orbitonasal foramen (orb.na.for.) forwards and slightly 

 outwards to the lacerate fossa (vide siopra, p. 388). 



The pituitary fossa or sella turcica (pit.fos.) is an almost spherical depression in the 

 middle of the cranial floor. It is bounded behind by a ridge, the dorsum sellse, 

 which curves forwards on each side and ends at the small oculomotor foramen 

 (oc.for.). In front the pituitary fossa is bounded by a transverse ])^'^pituitary ridge 

 (fig. '].Q,pr.pit.r.), anterior to which is a wide ledge, the optic platform, terminating 

 on either side in the optic foramen (op.for.). Almost vertically above the prepituitary 

 ridge is a similar transverse prominence, the pi'eoptic ridge (fig. 16, pr.op.r.), forming 

 the upper boundary of the optic platform and passing laterad into the tentorial ridge. 



The optic foramen is bounded behind by a vertical bar of bone separating it from 

 the apertures for the third nerve and the internal ophthalmic artery (fig. 16). The 

 oculomotor foramen [oc.for.), which is the uppermost of the two, is continued back- 

 wards by a groove placed sometimes just above, sometimes just below the continuation 



3 k2 



