CHAP, ii.] THE BRAIN. 1097 



started in afferent fibres, in those for instance of the sciatic nerve, 

 so affect the vaso-motor 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 vaso-motor centre in the 

 bulb. It is however exceedingly probable that the impulses which, 

 generated in sensory nerves, affect the vaso-motor centre are 

 impulses which, in the conscious animal, give rise to sensations 

 of pain ; in an intact animal changes in the vaso-motor centre 

 occasioned by the stimulation 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 indicating 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 



