Chap, ii.] THE BRAIK 803 



along the fibres of the lateral column, some having kept to the 

 same side of the cord, but more having crossed over to the oppo- 

 site side, before reaching the thoracic region. Though these 

 vaso-motor experiments have a certain value, inasmuch as the 

 results gained by them are more or less distinctly quantitative 

 and measurable, many objections may be urged against their 

 validity as affording a general proof of the course taken in the 

 cord by impulses-giving rise to sensations. They were conducted 

 on rabbits, animals low in scale and especially so perhaps in re- 

 spect to the spinal cord, they were limited to one region of the 

 cord, the observations were made immediately after the division 

 of the cord, before the immediate effects of the operation had 

 passed off; and further, it may be urged that impulses affecting 

 the vaso-motor centre may not be identical with those giving rise 

 to sensations. 



Many experiments have been made on dogs ; and if we con- 

 tent ourselves with making no distinction between the different 

 kinds of afferent impulses, and in the case of these animals it 

 would hardly be profitable to attempt to make a distinction, we 

 may say that the several experiments so far agree that they point 

 to the lateral columns as being the chief paths of afferent, sensory, 

 impulses, or to speak more exactly, to the passage of these im- 

 pulses being especially blocked by section of the lateral columns. 

 Some observers find that in the dog a section of the lateral column 

 on one side, or at least a hemisection of the cord, produces 'loss 

 of sensation' on the opposite side greater than on the same side, 

 or confined to the opposite side, and even accompanied by an ex- 

 altation of sensation, a hyperesthesia, on the same side. Other 

 observers again, and these certainly competent observers, find that, 

 in the dog, section of one side affects sensation on both sides, and 

 indeed chiefly on the same side. We may perhaps once more re- 

 peat the warning how difficult is the quantitative and qualitative 

 determination of sensations in such an animal as the dog; and may 

 remark that in all these cases of unilateral section the increased 

 blood supply due to failure of the normal vaso-constrictor tone 

 must influence the peripheral development of sensory impulses. 



In these experiments, as in those on voluntary movements, 

 it is most important to distinguish between immediate or tem- 

 porary and more lasting effects ; and observers have found that 

 the loss of sensation following a hemisection of the cord, like 

 the loss of voluntary movement, is temporary only, and event- 

 ually disappears, though the recovery is slower and less complete 

 than is the case with movements. As with voluntary movement 

 (§ 491) so with sensation, recovery, though less complete than 

 that of movement, is possible when a hemisection on one side 

 has been at a later date followed by a hemisection on the 

 other side. 



The experiments on monkeys are in like manner neither 



