648 



ANATOMY OF VERTEBRATES. 



corpus cavernosum, the urethra, with a corpus spongiosum, pro- 

 jecting and opening beneath the apex. In Perameles, Didelphys, 

 Phascolarctus, and Pkascolomys, the corpus spongiosum maintains 

 its character for a greater extent, and may be more distinctly 

 recognised as forming the proper wall of the urethral canal, which 

 sooner becomes superficial, and the glans penis is better defined. 

 In the Kangaroo and Potoroo, the s erectores penis,' fig. 503, d, d, 

 arise by a thin fascia from near the lower part of the symphysis 

 pubis, soon become fleshy, and increase in thickness as they pass 

 outward : each muscle then returns upon itself, at an acute bend, 

 to grasp the crus penis, and terminates in a strong tendinous ex- 

 pansion at the junction of the cavernous with the bulbous structures. 

 The f retractor penis,' figs. 503, 504, g, g, arises in the Kangaroo 

 from the middle of the sacrum, and divides into two muscles, 



behind the rectum, opposite 

 the dilated commencement 

 of the musculo-prostatic 

 part of the urethra; each 

 division diverges to the side 

 of the rectum, then passes 

 to the interspace between 

 the rectum and roots of the 

 penis, converging along the 

 lateral and posterior part of 



504 



the 



penis. 



to be inserted 



Male organs, Opossum, ccxxxv' 



with the opposite muscle at 

 the base of the glans. In 

 the Opossum and those Mar- 

 supials which, having a bi- 

 fid glans, enjoy, as it were, 

 a double coitus, there is a 

 6 levator penis,' fig. 504,/,/, 

 which is not present in the 

 Kangaroo. Each portion of this muscle takes its origin from 

 the fascia covering the crus penis, converges towards its fellow 

 above the dorsum penis, diminishing as it converges, and termi- 

 nates in a common tendon inserted into the upper part of the base 

 of the glans. There is another powerful muscle which, though 

 not immediately attached to the penis, must exert in all Marsu- 

 pials an important influence upon its erection. This is the external 

 ' sphincter cloacae : ' it is an inch and a half in breadth in the 

 Kangaroo, and half an inch in thickness ; from the back of the 

 termination of the rectum it passes over the anal glands and sides 



