992 THE CENTRAL NERVOUS SYSTEM 



a high protein content is easily understood, for the protoplasmic 

 structures, the nerve-cells, are situated in the grey matter. But 

 that the most important functions should have their seat in a tissue 

 containing only 14 to 19 per cent, of solids is surprising, and should 

 warn us that the water is no less significant a constituent of living 

 matter than the solids, and that it is not the mass of the solid 

 substances in a tissue which is the essential thing, but the whole 

 colloid complex, which cannot be constituted without the water. 



Fresh nervous tissues are alkaline to litmus, but become acid 

 soon after death. No change of reaction has been detected during 

 activity. 



That oxygen is used up during cerebral activity is certain, and 

 when the brain is coloured with methylene blue, by injecting it 

 into the circulation, any spot of it which is stimulated loses the 

 blue colour, the pigment being reduced. But if the animal is so 

 deeply narcotized that it does not respond to stimulation, the change 

 of colour does not occur. 



Cholin (p. 366), a substance which can be derived from lecithin, 

 is believed to represent one of the waste products of nervous activity. 

 Exceedingly small traces of it are present in normal cerebro-spinal 

 fluid, and in certain diseased conditions of the brain, as in general 

 paralysis, the quantity is said to be notably increased, indicating 

 an increased decomposition of lecithin. The fatty acid constituent 

 of lecithin is liberated in degenerating nerve, giving rise to the 

 reaction with osmic" acid (p. 797). Some writers assert that this 

 increase in the cholin can be used as a test to distinguish organic 

 nervous disease from that which is purely functional. But the 

 matter is in dispute. 



Cerebro-spinal Fluid. The cerebro-spinal fluid, which fills the 

 ventricles of the brain and the central canal of the cord, is con- 

 tinuous with that contained in the subarachnoid space through the 

 foramen of Magendie, an opening in the piece of pia mater that helps 

 to roof in the fourth ventricle. It is secreted in part by the cubical 

 cells covering the choroid plexus, a fold of pia mater which projects 

 into each lateral ventricle. Extracts of choroid plexus, when in- 

 jected intravenously, increase the rate of secretion. 



This action is dependent upon the presence of some substance in 

 the choroid plexus, which, however, is not a specific product of the 

 activity of the plexus, since extracts of the brain produce the same 

 effect. It may therefore be some product of the metabolism of the 

 brain which passes to the choroid plexus and stimulates secretion 

 by the epithelium. The substance is removed from the fluid by 

 filtration through a Chamberland filter, and is therefore probably 

 of high molecular weight. It is probable that variations in the rate 

 of secretion of the cerebro-spinal fluid by the choroid plexus are more 

 influential in governing the intracranial pressure than variations in 



