SKELETON. 



643 



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 maxilla and premaxillte), 

 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 

 and a more external porous 

 mastoid portion which is pro- 

 duced downwards into a mastoid 

 process in front of the par- 

 occipital process. From each /.{-, 

 periotic a tympanic bone ex- 

 tends outwards, swollen basally 

 into a tympanic bulla in which 

 the tympanum or drum of the 

 ear is stretched, and continued 

 around the external auditory 

 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 orbito-sphenoids, below by the presphenoid. 



Fig. 



279. — Under surface of rabbit's 

 skull. 



Front incisors; p.i., small posterior 

 incisors ; pmx., premaxilla (the lower part 

 of the reference line points to its palatal 

 process); mx., maxilla ; /«/.;,, third pre- 

 molar ; wz-3, third molar;/., jugal ; yr. , 

 supra-orbital ridge of frontal (on dorsal 

 surface) ; z.sg., zygomatic process of 

 squamosal; Sq., squamosal; T.6., tym- 

 panic bulla ; b.o., basioccipital ; />..*-., basi- 

 sphenoid ; pt., pterygoid ; pi., palatine. 



