258 THE PHYSIOLOGY OF REPRODUCTION 



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 withdrawn within an eversible fold of skin which constitutes a 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.^ 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 penis is withdrawn, and ventral to it when it is everted, is seen the 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 

 iirethral 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 Caviidae 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 contrivance is to 

 act as an exciting organ on the sexual structures of the female. 



In another rodent, the marmot, according to Gilbert^ the skin 

 which covers the os penis becomes torn away during the rutting 

 season, so that the bone projects freely beyond the end of the glans 

 and is then used as a stimulating organ. 



Structures which project from the penis, and are probably 

 employed as sexual irritants, are also found in the rhinoceros, the 

 tapir, and certain other animals. 



In the cat the glans is beset with callous retroverted papillae, 

 which no doubt serve the same function. They are present also in 

 the lion and tiger, but are of smaller size.^ 



Perhaps the most curious modifications presented by the 

 mammalian organ of copulation are those found in certain species of 



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

 Male Guinea-Pig," Jour, of Anat. and Pkys., vol. xxxii., 1898. 



2 Gilbert, " Das Os priapi der Saugethiere," Morph. Jahrhuch, vol. xviii. 



3 Owen, On the Anatomy of Vertebrates, vol. iii., London, 1868. 



