708 THE BRAIN. 



ing this matter, we may suppose that some of these convey impulses upward, 

 and others downward. 



If, as some maintain, the fibres of the ascending antero-lateral tract end 

 not in the cerebellum, but in the gray matter of the bulb, or higher up, we 

 have a fourth path for sensory impulses, which, after the primary relay in 

 the segmental gray matter, pass straight up to the bulb. 



593. How do experimental results and clinical histories accord with 

 such an anatomical programme? 



We may first call attention to an experiment, which, though somewhat 

 old, carried out on rabbits, and confined to one region only of the cord, the 

 lower thoracic, has nevertheless a certain value on account of its affording 

 more or less distinctly quantitative and measurable results. We have seen 

 ( 161) that afferent impulses started in afferent fibres, in those, for instance, 

 of the sciatic nerve, so affect the vasomotor centre in the bulb as to cause a 

 rise of blood-pressure, at least in an animal under urari. Those afferent 

 impulses must pass by some path or other from the roots which supply the 

 sciatic nerves with afferent fibres along the thoracic and cervical cord to the 

 bulb. If the path be blocked, the stimulation of the sciatic nerve will fail 

 to produce the usual rise of blood-pressure. Now in a rabbit the amount of 

 rise of blood-pressure following upon the stimulation of one sciatic nerve 

 with a certain strength of current having been ascertained, it is found that 

 a much less rise of blood-pressure or none at all follows the same stimulation 

 after division of certain parts of the cord in the mid or upper thoracic 

 region ; that is to say, the section of the cord has partially or completely 

 blocked the path of the afferent impulses. Further, the block is conspicuous 

 when the lateral column is divided, and is not increased by other parts of 

 the cord being divided at the same time ; when both lateral columns are 

 divided, the block is almost complete. And further, supposing one sciatic, 

 say the right, is the one which is stimulated, a block occurs both when the 

 lateral column of the same, right, side and when that of the crossed, left, 

 side is divided, but is greater when the division is on the crossed than when 

 it is on the same side. We may infer that the impulses, which reach the 

 lumbar cord by the roots of the sciatic nerve, travel up the cord, or give 

 rise within the lumbar cord to events which we may compare to nervous 

 impulses, and which travel up the cord in such a manner that in the lower 

 thoracic region they pass almost exclusively along the fibres of the lateral 

 column, some having kept to the same side of the cord, but more having 

 crossed over to the opposite side before reaching the thoracic region. 



This result was obtained in rabbits, and the experiment was carried out 

 in the lower thoracic region only ; the conclusions to be drawn from it hold 

 good for that animal only, and for that part only of its cord. Moreover, 

 the experiment only tests the path of such impulses as reach and affect the 

 vasomotor centre in the bulb. It is, however, exceedingly probable that the 

 impulses which, generated in sensory nerves, affect the vasomotor centre are 

 impulses which, in the conscious animal, give rise to sensations of pain ; in 

 an intact animal changes in the vasomotor centre occasioned by the stimula- 

 tion of sensory nerves are accompanied by signs of more or less pain. And 

 indeed this is confirmed by the fact that similar results were obtained when, 

 the experiment being conducted in a similar way, signs of pain instead of 

 variations in blood-pressure were taken as the tokens of the blocking of 

 impulses. Hence, assuming this, we may regard the experiment as indicat- 

 ing that the impulses which form the basis of painful sensations pass by the 

 lateral columns in the lower thoracic region of the cord of the rabbit, and 

 therefore, though 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 



