xi SKELETON 487 



like urinogenital aperture is called the vulva ; in the male 

 the aperture is smaller and is situated'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 differ- 

 entiated. 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.fo). 



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. 



