314 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXII. 



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 lamellae which divide its cavity into small 

 compartments. 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 variations of size, collapsing in the passive state of the organ, but 

 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 distension of the vessels, 

 and during the state of erection, give to the penis its condition of tension and firm- 

 ness. The same general condition of vessels exists in the corpus spongiosum 

 urethras, 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. 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 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, and derived reflexly 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 the penis is 

 no longer capable of erection. 



Erection is not complete, nor maintained for any time except when, together 

 with the influx of blood, the muscles mentioned contract, and by compressing the 

 veins^stop the efflux of blood, or prevent it from being as great as the influx. 



The circulation in the Lungs, Liver, Spleen, and Kidneys will be described in our 

 study of those organs. 



