168 HAND-BOOK OF PHYSIOLOGY. 



cranium was determined by the position in which the animal was placed 

 after death, the cerebral vessels being congested when the animal was sus- 

 pended with its head downward, and comparatively empty when the 

 animal was kept suspended by the ears. ' That, it was concluded, although 

 the total volume of the contents of the cranium is probably nearly always 

 the same, yet the quantity of blood in it is liable to variation, its increase 

 or diminution being accompanied by a simultaneous diminution or in- 

 crease in the quantity of the cerebro-spinal fluid, which, by readily 

 admitting of being removed from one part of the brain and spinal cord to 

 another, and of being rapidly absorbed, and 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 (Bur- 

 rows). And there can be no doubt that, although the arrangements of 

 the blood-vessels, to which reference has been made, ensure 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 cavity 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. Hence we may con- 

 sider that the cerebro-spinal fluid in the interior of the skull not only 

 subserves the mechanical functions of fat in other parts as & packing 

 material, but by the readiness with which it can be displaced into the 

 spinal canal, provides the means whereby undue pressure and insufficient 

 supply of blood are equally prevented. 



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, car- 

 bonates, alkaline and earthy phosphates, minute traces of urea, sugar, 

 sodium lactate, fatty matter, cholesterin, and albumen (Flint). 



2. 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 caver no sa 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 



