882 A MANUAL OF PHYSIOLOGY 



cant a constituent of living matter than the solirh, 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, 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. 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 

 continuous 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 increase the rate of secretion. Cerebro-spinal 

 fluid can easily be obtained in man by lumbar puncture with a 

 hypodermic needle sufficiently long to enter the subarachnoid 

 space in the spinal canal. The point usually selected for the 

 puncture is between the fourth and fifth lumbar vertebrae. 

 The normal pressure of the fluid is such that it trickles out by 

 drops, but in disease it is sometimes so high that it spurts out in 

 a steady stream. An examination of the fluid, especially for leuco- 

 cytes or bacteria, is of great diagnostic value in certain conditions. 

 Normally it is a thin, clear, watery fluid, faintly alkaline in 

 reaction to litmus, and with a specific gravity of about 1004 to 

 1007. It contains the ordinary salts, but more potassium than 

 sodium, unlike other body fluids ; a very small amount of protein 

 (globulin) usually about o-i per cent. and a little dextrose 

 (Nawratzki). Its composition is evidently different from that of 

 ordinary lymph. Only a few lymphocytes are present in health, 

 but in some diseases (as in general paralysis of the insane, tabes, 

 and cerebro-spinal syphilis) a marked increase occurs. In acute 



