CIRCULATION IX ERECTILE STRUCTURES. 183 



were there not provision made for the possible displace- 

 ment of some of the contents of the unyielding bony 

 case in which the brain is contained, there would be often 

 alternations of excessive pressure with insufficient supply 

 of blood. Hence we may consider that the cerebro-spinal 

 fluid in the interior of the skull not only subserves the 

 mechanical functions of fat in other parts as a packing 

 material, but by the readiness with which it can be dis- 

 placed into the spinal canal, provides the means whereby 

 undue pressure and insufficient supply of blood are equally 

 prevented. 



Circulation 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 corpus 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 lamella) 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, col- 

 lapsing in the passive state of the organ, but, for erection, 

 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 



