1116 THE CORTEX. [BOOK m. 



bolism of the cardiac substance ; the influence of external 

 circumstances, apart from those which provide an adequate supply 

 of proper blood, is wholly subsidiary and serves only to raise or 

 to lower the intrinsic changes from time to time, as occasion may 

 demand. And the analogy of the heart has perhaps led us to 

 exaggerate the part played in the brain by the like intrinsic 

 chemical metabolism. (We are here of course viewing the action 

 of the brain from the only stand-point admissible in these pages, 

 the purely physiological one ; but such a mode of treatment does 

 not prejudge other points of view.) Some writers use expressions 

 which seem to imply the conception that the nervous changes 

 forming the basis of the psychical and other processes of the brain 

 are chiefly the direct outcome of the chemical metabolism of the 

 grey matter and especially of the nerve cells. They speak of " the 

 discharge of energy" from these cells in the same way that we 

 can speak of the discharge of energy from a cardiac fibre. But, to 

 say nothing of the low rate of nervous metabolism as measured 

 in terms of chemical energy, we have no experimental or other 

 evidence of nervous substance in any part of the body being, like 

 the cardiac substance, the seat of an important metabolism 

 carried on irrespective of influences other than purely nutritive 

 ones. In the case of nerve cells interpolated along nerves 

 composed of fibres of the same kind, as in the sporadic ganglia, 

 all the instances where the nerve cells were supposed to initiate 

 active processes have, on examination, broken down ; as we have 

 seen, the ganglia of the heart do not supply the moving cause of 

 the heart beat. It is only in the central nervous system where nerve 

 cells, as part of grey matter, are found at the meeting of nerve- 

 fibres of different kinds, that we have any evidence of " discharge 

 of energy" from the cells. 



As we pointed out ( 597) in speaking of the spinal cord, the 

 discharge of efferent impulses from the central nervous system, 

 though it undoubtedly must have a certain chemical basis, namely, 

 the metabolism of the nervous substance, is, in the first line, 

 dependent on the advent of afferent impulses. But this, if true of 

 the spinal cord, is still more true of the brain, which receives or 

 may receive not only all the impulses which reach it through the 

 cord, but especially potent and varied impulses directly through 

 the cranial nerves. All life long the never ceasing changes of 

 the external world continually break as waves on the peripheral 

 endings of the afferent nerves, all lifelong nervous impulses, now 

 more now fewer, are continually sweeping inwards towards the 

 centre ; and the nervous metabolism, which is the basis of nervous 

 action, must be at least as largely dependent on these influences 

 from without, as on the mere chemical supply furnished by the 

 blood. 



We have developed this point because of the influence it 

 must have on our conceptions of the physiological processes taking 



