438 ZOOLOGY SECT. 



external surface is directed backwards and upwards, and elevated 

 in the middle into a shield-shaped prominence. The ex-occipitals 

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

 the somewhat oval prominence or condyle 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 styloglossus, 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 cerebral 

 nerves, the hypoglossal. 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 adult Rabbit united together to form the single 

 occipital bone. Articulating in front with the basi-occipital, but 

 separated from it by a plate of cartilage, 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 perforated 

 at about its middle by an oval foramen the pituitary foramen 

 and on its upper surface is a depression, the sella turcica, or 

 pituitary fossa (Fig. 1094, s.t.), in which the pituitary body rests. 

 In front of it is another median bone of laterally compressed form, 

 the pre-sphenoid, with which it is connected by cartilage, the removal 

 of which leaves a gap in the dried skull ; the pre-sphenoid forms 

 the lower boundary of the single large optic foramen (Fig. 1093, 

 opt.fo.) Connected laterally with the basi-sphenoid and pre-sphenoid 

 are two pairs of thin irregular plates, the ali-sphenoids (as.) behind 

 and the orbito- sphenoids (o. sph.) in front. The alisphenoids are 

 broad wing-like bones, each produced below into a bilaminate 

 process, the pterygoid process. A large foramen, the sphenoidal 

 fissure (sph.), situated between the basisphenoid and the alisphenoid 

 of each side, transmits from the interior of the skull the third 

 and fourth cerebral 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 

 (Fig. 1094, eth.), perforated by numerous small foramina for the 

 passage of the olfactory nerves. This cribriform plate forms a 

 part of a median vertical bone, the mesethmoid, the remainder of 

 which, or lamina perpendicularis, forms the bony part of the 

 partition (completed by cartilage in the unmacerated skull) between 

 the nasal cavities. Fused with the mesethmoid are two lateral, 

 thin, twisted bones, the ethmo-turbinals (e. tb), and with its inferior 

 edge articulates a long median bone, with a pair of delicate lateral 

 wings, the vomer (vo.). None of these, with the exception of the 

 cribriform plate, take any share in the bounding of the cavity 



