CUTANEOUS AND SOME OTHER SENSATIONS. 709 



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 interpretation 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, in- 

 deed, 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 the former. It is possible, for instance, that the section of 

 the thoracic lateral column inhibited, for the period during which the ex- 

 periment was carried out, the gray matter of the lumbar cord, and that the 

 block really took place in this gray 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 show 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 vasomotor 

 centre gain access to that centre without the help of the parts of the brain 

 above the bulb ; the existence of the vasomotor centre was made out ( 162), 

 by combining stimulation of a sciatic nerve with a series of operations con- 

 sisting in making successive transverse sections of the bulb from above 

 downward ; and it was not until the sections reached the vasomotor centre 

 that the blood-pressure effects of the sciatic stimulation were modified. 

 Hence, if the experiment be taken as showing that not only afferent im- 

 pulses affecting the vasomotor centre, but other afferent impulses also travel 

 by the lateral columns, it would also seem to show 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 vasomotor centre are alone considered. If the experi- 

 ment 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 doubtful view that the ascending antero-lateral tract ends in 

 the bulb, there is no continuous afferent tract in the lateral column ending 

 in the bulb ; the only definite continuous afferent tract in the lateral column 

 of which we have any clear knowledge, namely, the cerebellar tract, ends 

 not in the bulb but in the cerebellum. And if we attempt to get out of 

 the difficulty by supposing that those impulses at least which affect the 

 vasomotor centre, after travelling for some distance in the cerebellar tract, 

 leave that tract for some path leading to the bulb (and the cerebellar tract 

 does probably give off as well as receive fibres along its course), we practi- 

 cally admit that the experiment does not prove the existence of a continuous 

 path. 



A further difficulty is raised by the fact that, according to the interpreta- 

 tion which we are discussing, the "section of the lateral column breaks the 

 paths of what we may consider two kinds of impulses : those, the larger 

 number, which have already crossed from one side of the cord to the other, 



