I 84 THE CIRCULATION. 



fibrous membrane or sheath, with which it is connected, 

 limit the distension of the vessels, and, during the state of 

 erection, give to the penis its condition of tension and 

 firmness. The same general condition of vessels exists in 

 the corpus spongiosum urethrse, but around the urethra 

 the fibrous tissue is much weaker than around the body of 

 the penis, and around the glands there is none. The 

 venous blood is returned from the plexuses by compara- 

 tively small veins ; those from the glans and the fore part 

 of the urethra empty themselves into the dorsal vein of the 

 penis ; those from the corpus cavernosum pass into deeper 

 veins which issue from the corpora cavernosa at the crura 

 penis ; and those from the rest of the urethra and bulb 

 pass more directly into the plexus of the veins about the 

 prostate. For all these veins one condition is the same ; 

 namely, that they are liable to the pressure of muscles 

 when they leave the penis. The muscles chiefly con- 

 cerned in this action are the erector penis and accelerator 

 urinse. 



Erection results from the distension of the venous plex- 

 uses with blood. The principal exciting cause in the erec- 

 tion of the penis is nervous irritation, originating in the 

 part itself, or derived from the brain and spinal cord. The 

 nervous influence is communicated to the penis by the pudic 

 nerves, which ramify in its vascular tissue : and Guenther 

 has observed, that, after their division in the horse, the 

 penis is no longer capable of erection. It affords a good 

 example of the subjection of the circulation in an indivi- 

 dual organ to the influence of the nerves ; but the mode 

 in which they excite a greater influx of blood is not with 

 certainty known. 



The most probable explanation is that offered by Pro- 

 fessor Kolliker, who ascribes the distension of the venous 

 plexuses to the influence of organic muscular fibres, which 

 are found in abundance in the corpora cavernosa of the 

 penis, from the bulb to the glans, also in the clitoris and 



