THE NEKVOUS SYSTEM 319 



some particular group, owing to unrelieved pressure, produces 

 a cumulative effect. Another lesson ; that the condition of the 

 nervous system, and therefore its conductivity, is determined 

 at any given moment by the sensory impulses which are reach- 

 ing it. We cannot describe the effect of a bright light as pain, 

 yet it agitates the grey matter, altering its state, in the same 

 way as the nerve-inflow which we recognize as pain. A wet 

 rag on the forehead does not assuage a headache by cooling the 

 brain (cf. p. 106). The headache is " in the scalp." The cool 

 wet rag diminishes the dilation of the bloodvessels of the fore- 

 head, and quiets the impulses from the skin which are pouring 

 into a tract of grey matter pain-agitated by the influences 

 ascending a visceral nerve usually the vagus. 



It is necessary to warn the reader that a reversion to the old 

 idea of " conduction through grey matter " i.e., otherwise 

 than by a chain of neurones is unorthodox. It is set forth 

 here because it seems to the writer that the various phenomena 

 which have to be accounted for fit in best with the hypothesis 

 of a double path. If evidence of the anatomical possibility of 

 extra-neuronic conduction is asked for, it may be pointed out 

 that the chrome-silver and methylene-blue methods, upon 

 which our knowledge of neurones is based, do not, in the very 

 nature of the case, show that grey matter consists only of 

 neurones and their obvious branches. As they select par- 

 ticular elements of structure, we can never by their use alone 

 know what they fail to show. Attention may also be called 

 to the fact that the same staining process which reveals peri- 

 cellular nets (p. 301) shows also a structure resembling a net- 

 work in the substance which intervenes between them. Truly 

 the method is a rough one. It may well be thought that the 

 nitric acid used to fix the tissue may cause strange coagula- 

 tions with solution of uncoagulated substance ; but, as was re- 

 marked with regard to the pericellular nets, regular patterns 

 indicate architectural differentiation. But whether these nets do 

 or do not give hints as to the nature of the conducting medium, 

 there is no difficulty in finding sufficient material, after all the 

 substance entering into the formation of the conducting 

 neurones, as we imagine them, has been accounted for. Ex 

 hypothesi, the conducting material is provided by the fibrils 

 of the sensory nerves in their extensions beyond the limits to 



