The Skull as a Whole. 135 



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 foramina are 

 here confluent, there being a single aperture for the transmission 

 of the optic nerves. The posterior ventral boundary of this aper- 

 ture contains a broad groove, the sulcus chiasmatis, which lodges 

 in the natural condition the optic chiasma. 



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

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

 of the ethmoid bone, and serving for the transmission of the 

 divided olfactory nerves. Its median portion projects slightly into 

 the cranial fossa as a low ridge, the crista galli, which is interposed 

 between the tips of the olfactory bulbs. 



In the ventrolateral portion of the cranial cavity may be found 

 the internal openings of the foramina described above, namely, the 

 superior orbital fissure, the foramen lacerum, the jugular foramen, 

 and the hypoglossal canal. The superior orbital fissure is almost 

 ventral in position to the foramen opticum, and is connected, back- 

 ward with the foramen lacerum by a broad, groove, the sulcus 

 sphenoidalis, which lodges in the natural condition, the roots of 

 the fifth nerve. This groove continues to the medial surface of the 

 periotic bone, where it is bridged, over by the tentorium cerebelli. 



On the lateral wall of the posterior cranial fossa, and enclosed 

 by the compact, white, petrous portion of the periotic bone, 

 is a series of three apertures leading into its substance. One of 

 these, much larger than the remaining two, is the subarcuate 

 or floccular fossa (fossa subarcuata s. floccularis). It lodges in the 

 natural condition the flocculus, a small stalked appendage of the 

 cerebellum. Ventral to this fossa, and also somewhat in front of it, 

 a thin ledge of bone extends over an oval opening, the internal 



