258 THE CIRCULATION OF THE BLOOD 



The Vascular Nerves for the External Genital Organs. The vaso- 

 dilators for these organs arise from the second and third sacral nerves and 

 pass to the organs by the nervi erigentes and the pelvic plexus. They form 

 the second great exception to the region of general outflow of vascular nerves. 

 The constrictors, on the other hand, arise in the spinal nerves from the last 

 dorsal and first four lumbar. They run the same course as given in the table 

 for the pelvic viscera. 



The greatest variations in the quantity of blood contained at different 

 times in the external genital organs are found in certain structures which 

 contain what is known as erectile tissue. These organs, under ordinary cir- 

 cumstances, are soft and flaccid, but at certain times they receive an un- 

 usually large quantity of blood, become distended and swollen by it, and 

 pass into the state termed erection. Such structures are the corpora cavernosa 

 and corpus spongiosum of the penis of the male, and the clitoris in the female. 

 The nipple of the mammary gland in both sexes, and, according to some 

 authors, certain nasal membranes contain erectile tissue. 



The corpus cavernosum of the penis, which is the best example of an 

 erectile structure, has an external fibrous membrane or sheath. From the 

 inner surface of the sheath numerous fine lamellae project into the cavity, 

 dividing it into small compartments, like cells when they are inflated. Within 

 these cells there is a plexus of veins upon which the erectile property of the 

 organ mainly depends. The plexus consists of short veins with very close 

 interlacings and anastomoses with very elastic walls admitting of great varia- 

 tions in size. They collapse in the passive state of the organ, but are 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 distention 

 of the vessels and give to the organ its condition of tension and firmness. 

 The same general condition of vessels exists in the corpus spongiosum 

 urethrae, but the fibrous tissue around the urethra is much weaker than 

 around the body of the penis, while around the glans there is none. The 

 venous blood is returned from the plexuses by comparatively small veins; 

 all of which are liable to the pressure of muscles where 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 by a sudden influx of blood resulting from the action of the nervous 

 vascular reflexes. It is facilitated by the special muscular mechanism 

 which prevents the outflow of blood. 



The Vascular Nerves for the Trunk and Limbs. The skin and 

 muscles of the trunk receive their cutaneous and motor nerves by a seg- 

 mental arrangement in which the innervation is by bands corresponding 

 with the segments of the cord and the spinal nerves. It is much the same 



