CIRCULATION OF THE BLOOD. 169 



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 dila- 

 tation 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 con- 

 dition of tension and firmness. The same general condition of vessels 

 exists in the corpus spongiosum urethra?, 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 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 acceler- 

 ator 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, 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 as- 

 sist 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 large plexiform 

 arrangement of the veins, and such arteries as may admit, upon occasion, 

 augmented quantities of blood. 



(3, 4, 5.) The circulation in the Lungs, Liver, and Kidneys will be 

 described under those heads. 



