XI SKELETON 487 



In the female the slit-like Urinogenital aperture is called 

 the vulva ; in the male the aperture is smaller and is situ- 

 ated on the conical apex of a cylindrical organ, the penis, 

 which can be retracted within a fold of skin, the foreskin 

 or prepuce. On either side of the penis is an oval pouch 

 of the skin, the scrotal sac, not very apparent in young 

 animals, in each of which a spermary or testis is contained. 



Skeleton. The skeleton of the adult rabbit consists 

 almost entirely of bone, but it must be remembered that, 

 in addition to certain cartilages described below, all 

 articular surfaces are covered or lined by a thin layer of 

 cartilage, and that the various parts of the skeleton are 

 connected together by ligaments. 



In the skull, both replacing and investing bones (p. 43) 

 are much more numerous than in the frog, and the 

 structure of the entire skull is far more complicated and 

 highly differentiated. A posterior, relatively large 

 cranial region, in the side walls of which the auditory 

 capsules are embedded, can be distinguished from an 

 anterior, somewhat conical facial region, constituting 

 the skeleton of the snout (Fig. 132). Just behind the 

 junction of these two regions on either side is a large 

 orbit, separated from its fellow by a thin interorbital 

 septum, perforated by a foramen for the optic nerve 

 (opt. fa). 



At the sides of the foramen magnum are the two 

 rounded occipital condyles; the auditory apertures (aud. 

 me) are situated at the sides of the posterior part of the 

 cranium, and the external nostrils open at the anterior 

 end of the snout. Most of the bones remain more or 

 less distinct throughout life, and are in contact along 

 lines or sutures, many of which are wavy or zig-zagged : 

 others, again, become completely fused in the adult, so 

 that their limits are no longer distinguishable. 



