802 CUTANEOUS SENSATIONS. [BOOK in. 



(we may as urged above neglect the ties between the bulb and 

 the cerebellum). This, starting from the gracile and cuneate 

 nuclei and crossing at the interolivary layer, passes on through 

 the mid-brain to the optic thalamus with the cells of which 

 its fibres seem to make connections, though according to some 

 observers part of the fillet is continued, without relays, right to 

 the cerebral cortex. But if, as is stated, the destruction of the 

 two nuclei followed by complete degeneration of the fillet en- 

 tails no obvious loss of sensation, the afferent impulses passing 

 along it must be of a peculiar kind. Other fibres of the anterior 

 lateral columns, some at least of which, probably most, if not all, 

 we may regard as afferent in nature, pass into the reticular for- 

 mation of the bulb, and thence onward, probably more or less 

 by relays into the tegmentum. 



507. How do experimental results and clinical histories 

 accord with such an anatomical programme? 



We may first call attention to a somewhat old experiment. 

 We have seen, 153, that afferent impulses 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 stimula- 

 tion 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 stimu- 

 lated, 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. And 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. ' We may infer that the impulses, 

 which reach the lumbar cord by the roots of the sciatic nerve, 

 travel up the cord, or rather give rise within the lumbar cord 

 to nervous impulses, which travel up the cord in such a manner 

 that in the lower thoracic region they pass almost exclusively 



