422 ZOOLOGY SECT. 



lie at the sides of the opening, and each bears the greater part of 

 the somewhat oval prominence or condyh with which the corre- 

 sponding surface of the atlas or first vertebra articulates. Each is 

 produced below into a process called the par-occipital (par. oc.\ 

 closely applied to the tympanic bulla. At the end of this, imbedded 

 in the tendon of a muscle, the stylo- glossus, is a small bony rod, 

 the stylo-hyal. A small aperture, the condylar foramen, situated 

 below the condyle, is for the passage of one of the cranial nerves, 

 the hypo-glossal. The basi-occipital is a median plate of bone, almost 

 horizontal in position, which forms the floor of the most posterior 

 part of the cranial cavity ; it bears the lower third of the occipital 

 condyles. All these four bones of the occipital region are in the 



(/ -L C_J 



adult Rabbit united together to form the single occipital ooiu. 

 Articulating in front with the basi-occipital is a plate of bone, 

 also horizontal in position, which forms the middle part of the 

 floor of the cranial cavity. This is the basi-sphenoid ; it is per- 

 forated at about its middle by an oval foramen, and on its upper 

 surface is a depression, the sclla turcica, or pituitary fossa, in 

 which the pituitary body rests. In front of it is another median 

 bone of laterally compressed form, the presphenoid, with which it 

 is connected by cartilage, the removal of which leaves a gap in 

 the dried skull ; the presphenoid forms the lower boundary of the 

 single large optic foramen (opt.fo^). Connected laterally with the 

 basi-sphenoid and pre-sphenoid are two pairs of thin irregular 

 plates, the ali-sphcnoid (as.) behind and the orbito-sphenoid (o.sph.) 

 in front. The ali-sphenoids are broad wing-like bones, each pro- 

 duced below into a bilaminate process, the pterygM process. A 

 large foramen, the sphenoidal fissure, situated between the basi- 

 sphenoid and the alisphenoid of each side, transmits from the in- 

 terior of the skull the third and fourth cranial nerves, the first 

 and second divisions of the fifth, and the sixth nerves. 



The boundary of the anterior part of the brain case is com- 

 pleted by a narrow plate of bone, the cribriform plate of the 

 ethmoid, perforated by numerous small foramina for the passage of 

 the olfactory nerves. This cribriform plate forms a part of a 

 median vertical bone, the mcsetiimoid, the remainder of which, or 

 liimiiw perpend icul a ris, forms the bony part of the partition (com- 

 pleted by cartilage in the unmacerated skull) between the nasal 

 cavities. Fused with the mesethmoid are two lateral, thin, twisted 

 bones, the cthmo-turl)inals, and with its inferior edge articulates a 

 long median bone with a pair of delicate lateral wings, the vomer. 

 None of these, with the exception of the cribriform plate, take any 

 share in the bounding of the cavity of the cranium. Roofing over 

 the part of the cranial cavity, the walls and floors of which are 

 formed by the sphenoid elements, is a pair of membrane bones, the 

 parictats (_>.), and further forward another pair, the fronto.ls (/?'.). 

 The parietals are plate-like bones, convex externally, concave 

 internally, which articulate with the supra-occipital behind by a 



