606 CONDUCTION OF IMPULSES [BOOK in. 



In connection with this, a curious apparent contradiction 

 between the results of pathological observation and experimental 

 investigation may be mentioned. On the one hand pathological 

 observation clearly teaches that a limited disease of the cord 

 affects the conduction of volitional impulses much more distinctly 

 than it does that of sensory impulses. A segment of the cord may be 

 very largely diseased, causing a large or complete block to volitional 

 impulses, and may yet serve to conduct sensory impulses, which 

 however are in that case generally retarded as if the impulses were 

 making progress upwards by a roundabout and difficult route. On 

 the other hand, in the case of experiments, after various sections of 

 the cord, the recovery of voluntary power is generally more speedy 

 and more complete than that of distinctly conscious sensations, even 

 when both are primarily affected by the operation. The contradic- 

 tion may partly perhaps be explained by the difficulty, in the case 

 of animals, of any objective quantitative determination of sensation, 

 but not wholly. Both facts point to the possibility of both sensory 

 and volitional impulses making their way by changed paths under 

 changed circumstances. 



We may conclude our observations on this difficult but probably 

 pregnant topic by the following statement. While we appear to have 

 evidence that sensory and motor impulses are connected, at their 

 entrance into and exit from the cord with complicated mechanisms, 

 in which the grey matter undoubtedly, and possibly portions of the 

 posterior and anterior columns, are involved, the paths along the 

 cord are not clearly known. We have some reason to think that 

 they pass largely along the lateral columns, but probably not in a 

 direct straightforward manner, the whole cord being functionally a 

 series of mechanisms, for which the white matter supplies com- 

 missural connections. And the view that the paths may shift 

 according to circumstances is not without a certain support. 



As was stated above, after unilateral section of the spinal cord, 

 the sensation on the same side below the injury, so far from being 

 diminished or lost has in a certain number of cases, though by no 

 means always, been observed to be increased ; the parts are then 

 said to suffer from hyperaesthesia. Since the hypersesthesia appears 

 immediately after the operation, it cannot be due to any inflam- 

 matory process. Nor can it be explained as simply the result of 

 the increased supply of blood to the peripheral terminations of 

 the sensory nerves, caused by the section involving vaso-motor 

 tracts ; since the simple section of a vaso-motor tract, as when the 

 cervical sympathetic is divided, does not give rise to hypersesthesia. 

 Nor can we explain it as due to a one-sided hyperhsemia of the 

 spinal cord itself, for we have no evidence that such a state of 

 things is brought about. Since it lasts for a very considerable 

 time it cannot be due to any passing exciting effect of the operation. 

 It has been suggested that the section in such cases has removed 

 previously existing influences which descending the cord exercised 



