28O THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



small foreign body will, if situated in the region of the spinal 

 bulb, produce the gravest symptoms. For the centres which 

 control the vascular and respiratory systems are rendered anaemic 

 thereby. The cerebral hemispheres may, on the other hand, be 

 compressed to a large extent without causing a fatal result. The 

 major symptoms of compression arise so soon as any local increase 

 of pressure is transmitted to the spinal bulb and causes anaemia 

 there. 



In Erectile Structures. The instances of greatest variation in 

 the quantity of blood contained, at different times, in the same 

 organs, are found in certain structures which, under ordinary 

 circumstances, are soft and flaccid, but, at certain times, receive an 

 unusually large quantity of blood, become distended and swollen 

 by it, and pass into the state which has been termed erection. 

 Such structures are the corpora cavernosa and coi*pus 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 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 con- 

 sists 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 bloo'd 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 

 firmness. The same general condition of vessels exists in the 

 corpus spongiosum urethrae, 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 muscle's 

 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 



