THE REGULATING MECHANISM 183 



of protoplasm. It is doubtful if the protoplasm 

 of a neurone, say in the brain, can act for longer 

 than a few seconds without oxygen, and the 

 removal of waste matters. Hence there is 

 immediate loss of consciousness if the supply 

 of blood is cut off from the cerebrum, and if 

 the quality of the blood be altered by the 

 presence of even small amounts of poisons 

 the effect is quickly felt. Nervous matter is 

 also extremely sensitive to shocks or variations 

 of pressure. Thus a sudden concussion will 

 often produce unconsciousness. The activities 

 of the nervous system, and especially mental 

 activities, depend on the interplay between 

 grey matter and blood, and the limit of 

 adaptation as regards blood supply, quality 

 of blood, and temperature, is apparently 

 very small. Nearly all the other functions of 

 the body, in a sense, are working towards 

 the end of the adequate nutrition of the grey 

 matter. 



102. There are certain definite mechanisms 

 connected with nervous activity that must 

 now be noticed. Sometimes if a sensory nerve 

 is stimulated, there may be no sensation or 



