CHAP. IL] THE BRAIN. 1065 



time on the other side, be made at a higher level, this second 

 operation is followed by results similar to those of the first ; there 

 is loss of voluntary power on the side operated on, with some loss 

 of power on the crossed side, and as in the first case this loss of 

 power not only on the same but also on the crossed side may 

 eventually disappear. This shews among other things that the 

 recovery after the first operation was not due to the remaining 

 pyramidal tract doing the work of both. Further, the hemLsection 

 may be repeated a third time, the third hemisection being on 

 the same side as the first, with at least very considerable return 

 of power over both limbs. That is to say, under such abnormal 

 circumstances voluntary impulses may, so to speak, thread their 

 way in a zigzag manner from side to side along the mutilated cord 

 until they reach the appropriate spinal motor mechanisms. Such 

 an abnormal state of things does not however really militate 

 against the view that under normal circumstances volitional 

 impulses normally travel along the pyramidal tract; but it does 

 shew, what indeed has already been shewn by the phenomena of 

 strychnia poisoning, 586, that in the central nervous system the 

 passage of nervous impulses (using those words in the general 

 sense of changes propagated along nervous material) is not rigidly 

 and unalterably fixed by the anatomical distribution of tracts of 

 fibres; in all such discussions as those in which we are engaged 

 we must bear in mind that physiological conditions as well as 

 anatomical continuity are potent in determining the passage of 

 these impulses. 



664. When we reflect on the great prominence of the 

 pyramidal tract in the spinal cord of man as compared with that 

 of the dog, we may justly infer not only that the pyramidal tract 

 is under normal circumstances more exclusively the channel of 

 volitional impulses in man than in such lower animals, but also, 

 bearing in mind the discussion in a previous chapter, 591, 

 concerning the activities of the spinal cord of man, that the 

 potential alternatives presented by the spinal cord of the dog are 

 greatly reduced in that of man. And such clinical histories of 

 disease or accidental injury in man as we possess support this 

 conclusion. Lesions confined to one half of the cord, or even 

 lesions confined to the lateral column of one half, appeal* to lead 

 to loss of voluntary power on the same side, and the same side 

 only, in the parts below the level of the lesion ; and the same 

 symptoms have been observed to accompany disease limited 

 apparently to the pyramidal tract of one side. Moreover, though 

 cases of recovery of power have been recorded, we have not such 

 satisfactory evidence as in animals of the volitional impulses 

 ultimately making their way along an alternative route ; but here 

 the same doubts may be entertained as were expressed in dis- 

 cussing the reflex acts of the cord in man. 



When we say that the loss of voluntary power is seen on the 



