SKELETON OF THE RABBIT. 553 



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 maxUlce and 

 fremaxillcE. 



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 out- 

 wards, 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 between 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 maxillcs, 

 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 

 vomers beneath, the mesethmoid in the median line, while internally 

 there are several thin scroll-like turbinal bones. 



The lower jaw or mandible consists in adult life of a single bone or 

 ramus on each side, but this is formed around Meckel's cartilage from 

 several centres of ossification. Its condyle works in the glenoid cavity 

 of the squamosal. 



The hyoid lies between the rami of the mandible, in the back of 

 the mouth, and consists of a median "body," and two pairs of horns or 

 cornua extending backwards. 



The Appendicular Skeleton consists of the bones of the 

 limbs and the girdles which support these. 



The pectoral girdle, which supports the fore-limbs and is 

 itself attached by muscles and ligaments to the vertebral 

 column, virtually consists of one bone — the scapula — on 

 each side. For in the rabbit, as in all Mammals except 

 Monotremes, the coracoid, though a distinct ossification, 

 forms only a small process projecting from the anterior 

 margin of the glenoid cavity in which the head of the 

 humerus works. The clavicle is also much reduced in the 

 rabbit, being only about an inch in length and very slender. 

 It is a membrane-bone, and lies in the ligament between the 



