230 HANDBOOK OF PHYSIOLOGY. 



as readily effused, would serve as a kind of supplemental fluid to the 

 other contents of the cranium, to keep it uniformly filled in case of 

 variations in their quantity. And there can be no doubt that, although 

 the arrangements of the blood-vessels, to which reference has been made, 

 insure to the brain an amount of blood which is tolerably uniform, yet, 

 inasmuch as with every beat of the heart and every act of respiration 

 and under many other circumstances, the quantity of blood in the cav- 

 ity of the cranium is constantly varying, it is plain that, were there 

 not provision made for the possible displacement 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. 



Chemical Composition of Cerebro- spinal Fluid. The cerebro- spinal fluid is 

 transparent, colorless, not viscid, with a saline taste and alkaline reaction, and 

 is not affected by heat or acids. It contains 981-984 parts water, sodium 

 chloride, traces of potassium chloride, of sulphates, carbonates, alkaline and 

 earthy phosphates, minute traces of urea, sugar, sodium lactate, fatty matter, 

 cholesterin, and albumen (Flint). 



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 quan- 

 tity of blood, become distended and swollen by it, and pass into the state 

 which has been termed erection. Such structures are the corpora caver- 

 noxa 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 lamellae 

 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 variations 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 fibrous membrane or sheath with which it is connected, 

 limit the distention 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 



