164: HANDBOOK OF PHYSIOLOGY. 



an erectile structure, has an external fibrous membrane or sheath, and 

 from the inner surface of the latter are prolonged numerous, fine lamellae 

 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, collaps- 

 ing in the passive state of the organ, but, for erection, capable of an 

 amount of dilatation which exceeds beyond comparison that of the ar- 

 teries and veins which convey the blood to and from them. The strong 

 fibrous tissue lying in the intervals of the venous plexuses, and the ex- 

 ternal fibrous membrane or sheath with which it is connected, limit the 

 distention 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 ure- 

 thra 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 veins of the 

 penis; those from the 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 urinae. Erection results from the distention 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 influence is communicated 

 to the penis by the pudic nerves, which ramify in its vascular tissue: 

 and after their division in the horse, the penis is no longer capable of 

 erection. 



This influx of the blood is the first condition necessary for erection, 

 and through it alone much enlargement and turgescence of the penis 

 may ensue. But the erection is probably not complete, nor maintained 

 for any time except when, together with this influx, the muscles already 

 mentioned contract, and by compressing the veins, stop the efflux of 

 blood, or prevent it from being as great as the influx. 



It appears to be only the most perfect kind of erection that needs the 

 help of muscles to compress the veins ; and none such can materially 

 assist the erection of the nipples, or that amount of turgescence, just 

 falling short of erection, of which the spleen and many other parts are 

 capable. For such turgescence nothing more seems necessary than a 



