246 THE PHYSIOLOGY OF REPRODUCTION 



cartilaginous or bony support, the os penis, which is developed 

 especially in the region of the glans. It is particularly large in 

 the walrus. In the Cetacea the penis is often of enormous size 

 (six feet in length in some species), and terminates in a point, 

 but is otherwise normal. It can be withdrawn into the body 

 when not being used. In copulation, whales apply their ventral 

 surfaces to one another. 



In most Rodents and Marsupials the penis in the relaxed 

 state is withdrawn within an eversible fold of skin which con- 

 stitutes a dermal sac. When the penis is erected this sac is 

 everted, and forms its outer integument. Cole has described 

 the structure of the intromittent sac in the male guinea-pig, 

 which appears to be typical of many other Rodents. 1 Dorsal 

 to the urethral aperture when the penis is withdrawn, and 

 ventral to it when it is everted, is seen the entrance to the 

 intromittent sac. Lying in the cavity of the sac are two 

 horny styles. Two dorsal longitudinal folds are also noticeable. 

 These are the backward prolongations of the lateral lips of the 

 urethral aperture, the ventral lip consisting of corpus spongiosum 

 and separating the aperture of the urethra from that of the sac. 

 Attached to the base of the sac are two retractor organs which 

 consist of elastic fibres and erectile tissue, and are connected 

 at their other extremities with the integument of the penis. 

 The eversion of the sac is brought about by the erection of the 

 two longitudinal folds referred to above. The whole of the 

 sac is composed largely of erectile tissue, but the tissue of the 

 longitudinal folds is even more highly erectile than the rest of 

 the sac. The entire structure is provided with a very rich 

 nerve supply. When the penis is erect, and the sac everted, the 

 two horny styles are protruded externally to a considerable 

 length. Moreover, both the sac and the surface of the glans 

 are covered with sharp spine-like structures, while in some 

 species of Caviidre they are provided also with two sharp horny 

 saws which are appended to the sides of the penis farther back. 

 There can be little doubt that the purpose of this unique con- 

 trivance is to act as an exciting organ on the sexual structures 

 of the female. 



1 Cole, " On the Structure and Morphology of the Intromittent Sac of the 

 Male Guinea-pig," Jour, of Anat. and Phys., vol. xxxii., 1898. 



