RODENTIA. 



395 



wise in the beaver, only they are of much 

 smaller proportionate size. 



In the squirrel, Cowper's glands are repre- 

 sented by two large conical "bladders twisted 

 upon themselves, the summits of which are 

 evidently of a glandular nature, and are 

 divided internally into numerous small cells. 

 Each of these organs opens by a large orifice 

 into a cul-de-sac, which occupies the interior 

 of the bulb of the urethra, and which is pro- 

 longed into a canal, that, becoming gra- 

 dually narrower, opens into the urethra near 

 the angle formed by the bend of the penis. 

 The walls of the conical bladders, which con- 

 stitute the substance of these glands, contain 

 muscular fibres, which serve to constrict 

 their cavities. In the Alpine marmot and 

 in the boback, these glands present a similar 

 structure. 



In the rats they are of very large size and 

 of a pyriform shape, their substance being en- 

 veloped in an aponeurotic sheath. 



Penis. The penis in the Rodentia is dif- 

 ferently arranged in different genera. In the 

 Guinea-pig and the agouti, this organ, after 

 running forwards in the ordinary manner as 

 far as the anterior margin of the symphysis 

 pubis, bends back again upon itself beneath 

 the skin of that region towards the anus, so 

 that the opening of the prepuce is situated 

 very little in front of the anal orifice. Mus- 

 cular fibres, derived from the creraaster mus- 

 cles, are inserted into the penis near its curva- 

 ture ; and others, derived from the external 

 oblique muscle of the abdomen, are connected 

 with the same point. The former probably 

 contribute to effect the protrusion of the 

 penis from its sheath, whilst the latter draw 

 it back again into its concealment. 



In the marmot, the penis, when it arrives 

 in the sub-pubic region, does not bend back 

 again to approximate the anus, but curves 

 directly downwards ; in which position it is 

 retained by ligamentous attachments. 



In many genera of Rodents, as, for example, 

 in the rats, the voles, the dormice, the jerboa, 

 the hares, and the lagomys, the penis, after 

 issuing from the pelvis, does not run forwards 

 beneath the symphysis of the pubis, but passes 

 directly backwards towards the anus, imme- 

 diately in front of which the orifice of the 

 prepuce is situated (fig. 281. ). 



In most of the Rodentia the penis contains 

 a bone, imbedded in the substance of the 

 corpus cavernosum. But the most remark- 

 able part of the penis in the order before us, 

 is the glans, which in many species is armed 

 with such a formidable apparatus of spines, 

 saws, and horny spikes, that it must indeed be 

 a rather stimulating instrument of excitement. 



In the Alpine marmot it is conical, and 

 terminated by a sharp point, formed entirely 

 by the extremity of the os penis. On the right 

 of this point is situated the opening of the 

 urethra, and on the left there is a small but 

 deep cul-de-sac. 



In the common rat, the extremity of the 

 penis, in its relaxed state, resembles a second 

 prepuce, there being here a wide cavity exca- 



vated in the centre of the glands, enclosing a 

 bone, the extremity of which projects beyond 

 it, and is furnished with two small, cartila- 

 ginous, lateral appendages. Beneath this is 

 situated the cavity of the urethra. Most of 

 the genera allied to the rats, such as the 

 hamsters, the voles, the dormice, &c. f have 

 their penis constructed upon the same plan ; 

 but in some the surface of the glans is smooth, 

 whilst in others it is covered with papillae, or 

 studded with fine hairs. 



The glans penis of the beaver is cylin- 

 drical in shape, but flattened at its extremity, 

 which is studded with large papillae, the 

 orifice of the urethra being situated near its 

 centre. 



In the Guinea-pig, the penis is supported 

 by a flat and slightly curved bone imbedded 

 in its upper portion, which reaches as far as 

 the extremity of the glans above the canal of 

 the urethra. Behind and below the termi- 

 nation of the urethral canal is a wide pouch, 

 in the bottom of which are lodged two long 

 cartilaginous horns. This pouch, during 

 erection, is everted, so that the horns pro- 

 trude externally. Two tendons are connected 

 with the bottom of this pouch, which run 

 along the penis inferiorly. and are connected 

 with a thin layer of muscular fibres, derived 

 from the bulb of the urethra and the rami of 

 the corpora cavernosa. These tendons, either 

 by their own elasticity, or by the action of the 

 muscular fibres connected with them, serve to 

 invert the pouch and draw it back again 

 within the glans. The whole surface of the 

 glans is covered with corneous scales, which, 

 with the two horns above mentioned, give it a 

 formidable appearance. 



Fig. 285. 



Penis of the spotted Cavy ( Cadogenys subfuscd). 

 (Fred. Cuv.) 



Yet even this is an innocent weapon when 

 compared with the penis of the agouti and 

 other allied genera, which, besides containing 

 a pouch inclosing strong horny spikes like 

 that of the Guinea-pig, has the whole surface 

 of the glans covered with sharp recurved 



