678 



MAMMALIA. 



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 to a slight extent 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 maxilhs and prcmaxilhc] , 

 which in the rabbit is very in- 

 complete. 



Wedged in between the oc- 

 cipitals, the squamosals, and 

 the bones of the basisphenoid 

 region, there is on each side a 

 periotic bone surrounding the 

 internal ear. It ossifies from 

 three centres in the cartilaginous 

 auditory capsule, and consists of 

 a dense petrous portion enclos- 

 ing the essential part of the ear 



pm 3 



FIG. 333-- 



-Uncler surface of rabbit's 

 skull. 



and a more external porous 

 mastoid portion which is pro- 

 duced downwards into a mastoid 

 process in front of the par- 

 f.i., Front incisors; /.?'., small posterior occipital process. From each 

 incisors ; pni.-c., premaxilla (the lower part periotic a tympanic bone ex- 

 of the reference line points to its palatal } outwards, swollen basally 



process); ;;,.r., maxilla, />///. 3, third pre- . . ' . -- 



molar; 1,1.3, third molar; /., jugal ; jr., into a tympanic bulla in which 

 supra-orbital ridge of frontal (on dorsal the tympanum or drum of the 



surface); z.sq., zygomatic process of e ar is stretched, and continued 



squamosal; Sq.. squamosal; / .(>., tym- , , , ,. 



panic bulla; !>.<>.. basioccipital ; /..*., basi- around the external auditory 

 sphenoid ; /A, pterygoid ; pi., palatine. 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 maxilhc, 

 and above by \hzfrontals. The interorbital septum is formed above 

 and behind by the orbito -sphenoids, below by the presphenoid. 



