106 ANATOMY OF THE WOOD RAT 



side) of what several investigators, including Osgood, 

 have termed the corpus spongiosum, while others seem to 

 have confused it with the corpus cavernosum. I am not 

 aware that its true homology in this type of mammal has 

 ever been carefully established, and as it differs in important 

 respects from a portion of the true corpus spongiosum as 

 usually understood, I employ the term for this structure 

 with reservations. The tendency has too often been, while 

 investigating the urino-genital system of mammals, to 

 interpret all questionable structures in terms of human 

 or feline anatomy, labelling existing glandular masses as 

 ^'Cowper's glands 2 and 3," rather than to approach the 

 subject with the thesis that such are distinct glands with 

 special functions. 



To return to the subject, the two bulbs of the corpora 

 spongiosa vary somewhat in precise shape, and several 

 hundred per cent in size in the specimens of the three sub- 

 genera at hand. In the active Neotoma the interior is 

 spongy and highly congested, and they apparently open 

 into the body of the urethra each by a minute duct, situated 

 caudo-ventrad to the ducts of Cowper's gland. The medio- 

 ventral portion of each is invested by the fibers of a muscle 

 which may be homologous with the M. bulbocavernosus, 

 although its nomenclature must remain a matter of doubt 

 until the precise status of the bulbs shall have been investi- 

 gated. These fibers are defined with some difficulty in 

 animals that are sexually inactive. 



The two Cowper's glands are situated craniad and slightly 

 dorsad of the bulbs of the corpora spongiosa. In Neotoma 

 they shghtly exceed 5 nmi. in diameter, and in the other 

 two subgenera, are but little more than half that size. 

 Each opens by a duct several millimeters long into the 

 body of the urethra. The latter is invested by the M. 

 compressor urethrae, is much the largest in the Neotoma 

 at hand, and in those and the single Homodontomys that 



