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 ¢ympanic 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 ¢ympanzc 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 /achryma/s and the maxilla, 
and above by the frozéals. The interorbital septum is formed above 
and behind by the orbito-sphenoids, below by the presphenoid. 
Associated with the olfactory chambers are the zasal/s above, the 
vomers 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, ¢.g. of the maxz//a, and the development of slender rod-like 
processes from some of them, e.g. the sgwamosal, which help to keep 
the parts of the skull firmly connected. 
The lower jaw or mandzble 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 
sguamosal. 
_ The hyoid lies between the rami of the mandzble, 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” 
feces 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 
45 
