CHAP, ii.] THE BRAIK 775 



many experimenters the loss of voluntary power on the operated 

 side diminishes after a while, and that the animal if kept alive 

 and in good health long enough appears to regain almost full 

 voluntary power over the affected parts. In such cases, as in 

 other operations on the central nervous system, there is no re- 

 generation of nervous tissue ; the two surfaces of the section 

 unite by connective not nervous tissue, and the tracts which as 

 the result of the section degenerate downwards or upwards are 

 permanently lost. Hence even if we admit that in the intact 

 animal a voluntary movement is chiefly carried out by means of 

 efferent impulses passing along the pyramidal tract right down 

 to the motor mechanisms of the cord immediately connected 

 with the motor nerves, we must also admit that the 4 will ' under 

 changed circumstances can find other channels for gaining access 

 to the same mechanisms. 



It has been further observed that if in the dog a hemisection 

 be made at one level, for instance in the lower thoracic region of 

 the cord, and then, after waiting until the voluntary power over 

 the hind limb of that side has returned, a second hemisection, 

 this 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 crossed but also on the same side 

 may eventually disappear. This shews among other things that 

 the recovery after the first operation was not due to the remain- 

 ing pyramidal tract doing the work of both. Further, the hemi- 

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

 on the same side as the first, and in this case also there may be 

 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 appropri- 

 ate 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, 461, 

 that in the central nervous system the passage of nervous im- 

 pulses (using those words in the general sense of changes prop- 

 agated 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 con- 

 nections are potent in determining the passage of these impulses. 



492. 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 



