CHAP. IK] THE BRAIN. 1105 



685. We may here perhaps raise once more, and this time 

 more pointedly than before, the doubt whether we are justified 

 in assuming, as we generally do assume, that the events which 

 take place in the fibres connecting relays of grey matter within 

 the central nervous system, are exactly the same as those which 

 take place in the fibres of nerves outside the central system, 

 during the passage of what we call a nervous impulse. Most 

 of our knowledge of a nervous impulse has been gained by the 

 study of the motor nerve of a muscle-nerve preparation. Our 

 knowledge of the processes in afferent nerves is much more 

 imperfect; but there are many facts which at least suggest that 

 the molecular events constituting an afferent impulse along an 

 afferent nerve are different from, and probably more complicated 

 than, those constituting an efferent impulse along an efferent nerve. 

 And, with regard to the processes taking place in fibres within 

 the central nervous system we have hardly any exact experimental 

 knowledge at all. It has been maintained by many observers that 

 not only the grey matter but also the tracts of white matter in 

 the spinal cord, while they are capable of conveying impulses in 

 one direction or the other, are incapable of being so excited by 

 artificial stimuli as to generate new impulses. These observers 

 maintain that, when movements or signs of sensation follow the 

 direct stimulation of various parts of the cord, the effects are due 

 to issuing motor fibres or entering sensory fibres having been 

 stimulated, and not to a stimulation of the intrinsic substance of 

 the parts themselves ; they propose accordingly to call these parts 

 " kinesodic " and " sesthesodic " respectively, that is to say, serving 

 as paths for motor or sensory impulses without being themselves 

 either motor or sensory. The evidence on the whole goes to shew 

 that this view is a mistaken one, that the various tracts of the 

 spinal cord, like the pyramidal tract and indeed other parts of the 

 brain, are excitable towards artificial stimuli. The question 

 cannot, however, be considered as definitely closed ; and the very 

 fact that it has been raised illustrates the point on which we are 

 now dwelling. We may further quote, in similar illustration of 

 the same point, the following remarkable fact which was observed 

 in the series of experiments referred to in 663 on the effects of 

 repeated hemisection of the spinal cord in dogs. The animal had 

 partially recovered voluntary movements in his hind limbs after a 

 third hemisection of the thoracic cord, and yet when, at his death, 

 a strong tetanizing current was directed through the bulb and 

 cervical cord, no movements of the hind limbs followed : the 

 impulses started by artificial stimulation could not pass the bridge 

 which sufficed for volitional impulses of natural origin. It is not 

 too much to say that our experimental knowledge as to the events 

 which accompany the activity of the structures within the central 

 nervous system is almost entirely limited to the recognition of the 

 " currents of action " referred to in 657. We are already going 



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