1064 VOLITIONAL IMPULSES IN THE CORD. [BOOK m. 



elusion at which we have arrived, namely, that 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 voluntary 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 attempted to divide or 

 otherwise cause a 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 movement 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 concomi- 

 tant interference with sensations concerning which we shall speak 

 presently, the loss of voluntary movement is not absolutely con- 

 fined 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 probably one of the effects of 

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

 indeed appears soon to pass away. But taking into consideration 

 what was said above concerning the effects of removing cortical 

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



