178 



PHYSIOLOGY OF XEUVE 



[CH. XV. 



The above figures relate to the peripheral portions of the nerves. Noll has 

 shown that the phosphorised material protagon also diminishes somewhat in the 

 central ends of cut nerves due to " disuse atrophy." 



Heat contraction of nerve. A nerve, when heated, shortens ; this shortening 

 occurs in a series of steps which, as in the case of muscle, take place at the coagula- 

 tion temperatures of the proteids present. The first step in the shortening occurs 

 in the frog at about 40, in the mammal at about 47, and in the bird at about 52 C. 

 The nerve is killed at the same temperatures. 



Cerebro-spinal fluid. This plays the part of the lymph of the central nervous 

 system, but differs considerably from all other forms of lymph. It is a very watery 

 fluid, containing, besides some inorganic salts similar to those of the blood, a trace 

 of proteid matter (globulin) and a small amount of sugar. It contains the merest 

 trace of choline ; but this is not devoid of significance, for this fact taken in con- 

 junction with another namely, that physiological saline solution will extract from 

 perfectly fresh nervous matter a small quantity of choline shows us that lecithin 

 and protagon are not stable substances, but are constantly breaking down and 

 building themselves up afresh ; in fact, undergoing the process called metabolism. 

 This is most marked in the most active region of the brain viz., the grey matter. 



