ON THE MAMMALIAN NERVOUS SYSTEM. 



327 



one cortex (i.e., lower limb focus) excited for a definite period, an electrical excitatory 

 variation is produced in the cord, the amount of which is indicated by the following 

 average of several observations. 



The brain of the Monkey was stimulated for a shorter time, and with far weaker 

 excitation than that used for the Cat, in consequence of our desire not to make the 

 cortex hyperexcitable. Even then the excitatory effect in the Monkey is nearly as 

 large as that in the Cat. The truth of this position is also evidenced by numerous 

 observations we have made under other circumstances, the Cat always requiring a 

 stimulus, which in the Monkey would have evoked a far higher excitatory effect. As 

 with all results obtained by the method of averages, no doubt this point would repay 

 accumulation of observations, since, although no absolute value can be given to the 

 figures, they seem to show specific differences. With the cord divided longitudinally, 

 and the excitatory effect observed in each half, this absolute difference apparently 

 disappears (see p. 354). 



Before proceeding further, it is essential that we should indicate the experimental 

 evidence on which we base our interpretation of the electrical changes in the cord. 



That the changes are due to the discharge of excitatory impulses from the cortex is 

 clear, but it may be asked what evidence apart from the parallelism discussed on 

 p. 324 there is to show that they are the concomitants of the passage of actual nerve 

 impulses through the portion of exposed cord, and not the results of conduction from 

 the electrical changes evoked in all the surrounding muscles which are thrown into 

 spasm. 



The evidence which shows that the change is one localised in the nerve fibres of the 

 exposed cord may be grouped as follows : 



(1.) The position of the electrodes being arranged to ensure isolation, as described 

 in Chapter III., p. 295, this isolation is sufficiently complete to allow of the surrounding 

 muscles being thrown into activity without any electrical change in the exposed 

 tract being evident in the galvanometer. 



(2.) In observations made in the Monkey with the freshly exposed cortex, the 

 initial and immediately succeeding stimulations of the lower limb area evoked effects 

 which, with the strength of the stimulus employed, were so exactly circumscribed 

 that, since the cord was divided and the lower limbs thus cut off, no demonstrable 



