& 



774 VOLITIONAL IMPULSES IN THE COED. [Book hi. 



their usual part. But we may pass from such cases as these 

 through others, until we come to cases where a skilled move- 

 ment which has been learnt and practised by the working of an 

 intelligent will, may continue to be carried out under circum- 

 stances which seem to preclude the intervention of any conscious 

 will at all ; and the transition from one case to another is so 

 gradual, that it is impossible to suppose that there has been any 

 shifting of the machinery employed for carrying out the move- 

 ment. So that a volitional origin is not an essential feature of 

 these so-called voluntary movements, and the machinery of the 

 motor cortex and pyramidal tract is available for other things 

 than pure volitional impulses. 



§ 491. The preceding discussion will enable us to be very 

 brief concerning a question which has from time to time been 

 much discussed, and which has acquired perhaps factitious im- 

 portance, viz. the question as to how volitional impulses leading 

 to voluntary movements travel along the spinal cord. The con 

 elusion at which we have arrived, namely, th^it in the normal 

 carrying out of voluntary movements the chief part is played by 

 efferent impulses passing along the pyramidal tract, carries with 

 it the answer that volitional impulses travel in the spinal cord 

 along the pyramidal tract. 



In the dog, in which the whole pyramidal tract crosses at the 

 decussation of the pyramids, we should expect to find that a 

 break in the pyramidal tract of one side of the cord at any point 

 along its length caused loss of voluntaiy movement on the same 

 side below the level of the break. And experiments as far as 

 •they go support this view. No one it is true has so far succeeded 

 in dividing or otherwise causing to break in the pyramidal tract 

 alone, leaving the rest of the cord intact ; and indeed, even if 

 an injury were limited to the area marked out as the pyramidal 

 tract, fibres other than pyramidal fibres would be injured at the 

 same time, since the tract is never a ' pure ' one. But it has 

 been found that a section of a lateral half of the cord, a lateral 

 hemisection, or a section limited to the lateral column of one 

 side has for one of its principal effects loss of voluntary move- 

 ment on the same side in the parts supplied by motor nerves 

 leaving the cord below the level of the section. We say 'one 

 of its principal effects ' because, besides the concomitant inter- 

 ference with sensations concerning which we shall speak pres- 

 ently, the loss of voluntary movement is not absolutely confined 

 to the same side ; there is some loss of power on the crossed 

 side, at least in a large number of cases. We must not lay stress 

 on this crossed paralysis because it is possibly one of the effects 

 of the mere operation, not a pure 'deficiency' phenomenon, and 

 indeed appears soon to pass away. But taking into considera- 

 tion what was said above concerning the effects of removing 

 cortical areas, it is important to note that in the experience of 



