CHAP. X.] THE NERVOUS TISSUES. 447 



Observation of the living organism also teaches us that the proportion 

 of oxygen which is supplied to certain of the central organs influences 

 their activity in a remarkable manner; thus the activity of the 

 respiratory centre in the medulla is affected chiefly by the amount 

 of oxygen of the blood which traverses it. Again, an adequate 

 supply of oxygen to the brain appears to be a condition essential 

 to the proper exercise of the mental functions, and it is probably 

 in consequence of deprivation of oxygen that the moment blood 

 is cut off from the brain, as by ligaturing or compressing some of 

 the large arteries supplying it, all mental acts cease. When, however, 

 we direct our inquiries to the nature of the processes which have 

 their seat in the nerve cells we are obliged to conclude that we are 

 yet altogether in the dark. 



The nerve fibre is much less directly influenced by a supply or 

 absence of oxygen than the central organs, and it is probably for 

 this reason, amongst others, that it survives, even in warm-blooded 

 animals, after the brain and spinal cord have ceased to manifest any 

 signs of vitality. 



The only change of a chemical nature which has been proved 

 to occur in nerves as a result of long continued activity, or at death, 

 is a change in the reaction of the axis cylinder, which from an alkaline 

 changes to an acid reaction. The grey matter of the brain having an 

 acid reaction even during life *, no change can be observed to occur at 

 death. 



When nerve fibres are cut off from their connection with certain 

 nerve cells, whilst the life of the animal is preserved, they gradually 

 undergo a fatty degeneration which affects the axis cylinders and 

 ultimately leads to an abolition of their power to act as conducting 

 organs. 



1 Gscheidlen, "Ueber die Reaction der nervosen Centralorgane. " Pfluger's Archiv, 

 Vol. vni. p. 171. 



