CIRCULATION IN ERECTILE STRUCTURES. 153 



come distended and swollen by it, and pass into the state 

 which has been termed erection. Such structures are the cor- 

 pora cavernosa and corpus spongiosum of the penis in the 

 male, and the clitoris in the female; and, to a less degree, the 

 nipple of the mammary gland in both sexes. The corpus 

 cavernosum penis, which is the best example of an erectile 

 structure, has an external fibrous membrane or sheath ; and 

 from the inner surface of the latter are prolonged numerous 

 fine lamellse which divide its cavity into small compartments 

 looking like cells when they are inflated. Within these is 

 situated the plexus of veins upon which the peculiar erectile 

 property of the organ mainly depends. It consists of short 

 veins which very closely interlace and anastomose with each 

 other in all directions, and admit of great variation of size, 

 collapsing in the passive state of the organ, but, for erection, 

 capable of an amount of dilatation which exceeds beyond 

 comparison that of the arteries and veins which convey the 

 blood to and from them. The strong fibrous tissue lying in 

 the intervals of the venous plexuses, and the external fibrous 

 membrane or sheath with which it is connected, limit the dis- 

 tension 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 

 glans there is none. The venous blood is returned from the 

 plexuses by comparatively 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 concerned in this 

 action are the erector penis and accelerator urinse. 



Erection results from the distension of the venous plexuses 

 with blood. The principal exciting cause in the erection of 

 the penis is nervous irritation, originating in the part itself, or 

 derived from the brain and spinal cord. The nervous in- 

 fluence is communicated to the penis by the pudic nerves, 

 which ramify in its vascular tissue: and Guenther has ob- 

 served, 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 individual organ to the 



