The Skull as a Whole. 81 



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 aperture 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 hypo- 

 glossal canal. The superior orbital fissure is almost ventral in position 

 to the foramen opticum, and is connected backward 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 subareuata 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 open- 

 ing, the internal aperture of the facial canal (canalis facialis), which 

 serves for the transmission of the seventh cranial (facial) nerve. Imme- 

 diately behind and below this aperture is the opening of the internal 

 acoustic meatus (meatus acusticus internus) for the transmission of the 

 eighth cranial (acoustic) nerve. The two apertures tend to be enclosed 

 by a shallow bony ridge, largely formed by the projecting ledge described 

 above, and resembling superficially the complete common tube repre- 

 sented by the internal acoustic meatus of the human skull. 



The facial portion of the skull is largely formed by the investing 

 bones of the upper jaw, palate, and mandible, but it encloses also the 

 entire olfactory region of the primary skull, including the nasal fossae 

 and associated turbinal bones. The upper jaw — the maxilla of the 

 human skull— is formed of two primary, and, in the rabbit, separate, 

 elements, the maxilla and premaxilla. They together form the greater 

 portion of the facial region — in the adult condition also a large portion 

 of the lateral walls of the nasal fossae — and bear in a ventrolateral 

 position low alveolar- processes (processus alveolares) for the sockets or 

 alveoli of the incisor and cheek-teeth. The maxilla bears the anterior 



