1098 CUTANEOUS SENSATIONS. [BOOK in. 



this is a further assumption, by the same columns along the 

 whole length of the cord. We further may infer that while some 

 of the impulses keep to the same side of the cord, others, and 

 indeed the greater number, cross to the opposite side. 



These conclusions entail assumptions, but the main interpreta- 

 tion of the whole experiment entails a still greater assumption. 

 The testing of the influence of the sciatic stimulation was carried 

 out soon after the section of the cord, and yet we have assumed 

 that the block of the impulses was due to a pure deficiency 

 phenomenon, the absence of a usual path. But we' have no 

 right to do this. It is possible that the section produced, in 

 some way or other, a depressing or inhibitory effect lower down 

 in the cord, affecting structures other than the lateral columns ; 

 all our experience indeed of the effects of operations on the cord 

 would lead us to expect this. It is further possible that a section 

 of the lateral column might produce this depressing effect, while 

 sections of other parts did not, or might produce more effect than 

 they could. It is possible for instance that the section of the 

 thoracic lateral column inhibited, for the period during which the 

 experiment was carried out, the grey matter of the lumbar cord 

 and that the block really took place in this grey matter. Until 

 the uncertainties thus attending the interpretation are removed 

 the experiment is not valid as a proof that the lateral columns 

 are the paths of afferent impulses ; it would, however, still 

 serve to indicate that the afferent impulses reaching the cord 

 along the sciatic nerve crossed over to a large extent before they 

 came under the influence of the inhibition, since we have no 

 evidence to shew that such an inhibitory action of the section 

 would be exerted chiefly on the crossed side. 



Again, we have seen that the afferent impulses affecting the 

 vaso-motor centre gain access to that centre without the help of 

 the parts of the brain above the bulb ; the existence of the vaso- 

 motor centre was made out, 176, by combining stimulation of a 

 sciatic nerve with a series of operations consisting in making 

 successive transverse sections of the bulb from above downwards ; 

 and it was not until the sections reached the vaso-motor centre 

 that the blood-pressure effects of the sciatic stimulation were 

 modified. Hence if the experiment be taken as shewing that not 

 only afferent impulses affecting the vaso-motor centre, but other 

 afferent impulses also travel by the lateral columns, it would also 

 seem to shew that these other impulses pass in like manner to the 

 bulb, and gain access to the cortex through the bulb. This 

 increases a difficulty which presents itself even when the afferent 

 impulses affecting the vaso-motor centre are alone considered. 

 If the experiment means anything, it means that the impulses 

 having in some way or other reached the lateral column, travel up 

 that column by some continuous path, and indeed is generally 

 taken as having that meaning. But if we put aside the very 



