THE SKULL. 



663 



basi-occipital ; the median basisphenoid which lodges the pituitary body 

 in a dorsal depression called the sella turcica ; the paired alisphenoids 

 fused to the sides of the basisphenoid ; the median presphenoid which 

 forms the lower margin of the optic foramen between the two orbits ; 

 the paired orbitosphenoids, fused to the presphenoid, sutured to the 

 alisphenoids and squamosals, and surrounding the optic foramen ; the 

 vertical pterygoids attached at the junction of basisphenoid and alisphen- 

 oids ; the partly vertical palatines, united above to the presphenoid and 

 behind to the pterygoids and alisphenoids, separating the posterior nasal 

 passages from the orbits, and uniting in front to form the posterior part 

 of the bony palate ; the median vertical mesethmoid cartilage extending 

 in front of the presphenoid, separating the two nasal cavities, posteriorly 



ossified and expanded into the sieve- 

 like cribriform plates through the 

 apertures of which the branches of 

 the olfactory nerves pass to the 

 nose ; the paired vomers along the 

 ventral edge of the mesethmoid ; 

 and lastly, the anterior bony palate 

 formed from inward extensions of 

 maxilla and premaxillce. 



Wedged in between the occipitals, 

 the squamosals^ and the bones of 

 the basisphenoid region, there is on 

 each side a periotic bone surround- 

 ing the internal ear. It ossifies from 

 three centres in the cartilaginous 

 auditory capsule, and consists of a 

 dense petrous portion enclosing the 

 essential part of the ear and a more 

 external porous mastoid portion 

 which is produced downwards into 

 a mastoid process in front of the 

 paroccipital process. From each 

 periotic a tympanic bone extends 

 outwards, swollen basally into a tym- 

 panic bulla in which the tympanum 

 or drum of the ear is stretched, and 

 continued around the external audi- 

 tory meatus. From an aperture be- 

 tween the tympanic and the periotic 

 the Eustachian tube passes to the 

 pharynx. Stretching from the tympanum to the fenestra ovalis of the 

 inner ear is the chain of minute ear ossicles, the three links of which 

 malleus, incus, and stapes possibly correspond respectively to the 

 articular, the quadrate, and hyo-mandibular or columella of most other 

 Vertebrates. 



The orbits are bounded anteriorly by the lachrymals and the maxilla, 

 and above by the frontals. The interorbital septum is formed above 

 and behind by the orbitosphenoids, below by the presphenoid. 



Associated with the olfactory chambers, are the nasals above, the 



Na. 



Pmx. 



FIG. 231. Dorsal view of 

 Rabbit's skull. 



S.O., Top of supra-occipital ; //., 

 interparietal ; T., tympanic ; Pa., 

 parietal ; Sq., squamosal ; Fr., frontal ; 

 /. jugal ; Na., nasal ; Pmx., pre- 

 maxilla. 



