600 CONDUCTION OF IMPULSES. [BOOK in. 



But admitting this, there still remains the question, How do 

 volitional and sensory impulses travel along the cord, between the 

 brain on the one hand and the grey matter belonging to this 

 or that nerve root on the other ? 



Our information concerning the conduction of impulses along 

 the spinal cord is derived partly by anatomical deduction, partly 

 from experiment and partly from pathological observation. These 

 several methods have their advantages and disadvantages. We 

 have just now brought forward a very general anatomical de- 

 duction. More detailed inferences are afforded by the Wallerian 

 method (see p. 484). When the spinal cord is diseased or injured at 

 any point, tracts of degenerated fibres may at times be traced on 

 the one hand upwards towards the brain, or on the other down- 

 wards in a peripheral direction. The former may be taken as 

 being sensory or afferent, and the latter, together with tracts of 

 degeneration in the cord which are found associated with disease of 

 the brain, as motor or efferent. Again, when the development of 

 the spinal cord is studied, it is found that the fibres of different 

 tracts assume their medullary sheaths at different times; and by 

 this means the longitudinal fibres of the cord may be differentiated 

 into tracts having different terminal connections, some tracts being 

 thus traced into the crura cerebri, others into the cerebellum, while 

 others appear to terminate in the medulla oblongata, or both to 

 begin and end in the cord itself. These different distributions 

 obviously suggest different functions. But all such anatomical de- 

 ductions must here, as elsewhere, be received with caution. 



When experiments are used as a means of inquiry, we 

 are met with the danger of confounding the immediate and 

 temporary effects of the operation, such as those produced by 

 shock, with the more real and lasting effects. It is difficult too in 

 such cases to determine the existence of sensations, and to dis- 

 tinguish between reflex and purely voluntary movements. The 

 difficulty of recognizing, and especially of quantitatively estimating 

 the value of, signs of sensation has however been met by an 

 ingenious use of variations in blood-pressure. We have seen that, 

 at all events in an animal under urari, afferent impulses occasion a 

 rise of blood-pressure. If, having determined the amount of rise 

 due to a definite stimulation of a sensory nerve, such as the 

 sciatic, we make an incision into the spinal cord of the dorsal 

 region, dividing for instance part of the lateral column on one side, 

 and afterwards find that the same stimulus applied in the same 

 way to the sciatic nerve, leads to a greatly diminished rise of 

 blood-pressure, we are justified in inferring that the afferent 

 impulses affecting blood-pressure are largely conducted through the 

 part of the lateral column which has been divided. We may 

 thus obtain a definite measure, in millimetres of mercury, of the 

 effect produced by the injury. On the other hand, the value of 

 this precise measurement is diminished by the doubt whether we 



