SKELETON. 705 



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 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 maxilla, 

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

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



Associated with the olfactory chambers are the nasals above, the 

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

 there are several thin scroll-like turbinal bones. As special characters 

 of the skull should be noted the incomplete ossification of certain of the 

 bones, e.g. of the maxilla, and the development of slender rod-like 

 processes from some of them, e.g. the squamosal, which help to keep 

 the parts of the skull firmly connected. 



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 on 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. 



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 all Mammals, except Monotremes, the 

 coracoid is vestigial. It is represented by an " epicoracoid " 

 process overhanging the edge of the glenoid cavity in which 

 the head of the humerus works, and there is also in 

 some cases a small independent ossification (coracoid or 

 metacoracoid) on the ventral surface of the glenoid cavity. 

 The clavicle is much reduced in the rabbit, being only 

 about an inch in length and very slender. The triangular 

 scapula has a prominent external ridge or spine, continued 

 ventrally into an acromion with a long metacromion pro- 

 cess. The scapula is usually strong and the clavicle is 



