714 THE BRAIN. 



may not be the mere block due to loss of continuity, but some action on the 

 gray matter with which the fibres are connected, whereby that gray matter 

 fails of its usual functions and ceases to carry onward the sensory impulses 

 reaching it from below ; it is also possible that this or that lesion of disease 

 may, directly or indirectly, affect particular parts of the gray matter or 

 affect the gray matter in a particular way, so that a certain kind of sensory 

 impulse and none other is blocked. On the other hand, we have reason to 

 think that the rate at which impulses travel along the gray matter is very 

 slow compared with that along nerve-fibres ; and in the struggle for life 

 rapidity of transmission of nervous impulses is of great importance. Hence 

 the view that the internuncial fibres intervene has more to commend it ; it 

 is, moreover, to a certain extent supported by clinical histories. But, if we 

 accept this view, we must at the same time admit that, in animals at least, 

 the lines provided by the internuncial tracts are not rigid that within limits 

 and under circumstances alternative routes are possible. 



597. We may here, perhaps, raise once more, and this time more point- 

 edly than before, the doubt whether we are justified in assuming, as we 

 generally do assume, that the events which take place in the fibres connect- 

 ing relays of gray 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 \ve 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 af- 

 ferent 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 \vithin the central nervous system, we 

 have hardly any exact experimental knowlege at all. It has been main- 

 tained by many observers that not only the gray matter, but also the tracts 

 of white matter in the spinal cord, while they are capable of conveying im- 

 pulses 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 sen- 

 sory 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 show that this view is a mis- 

 taken one ; that the various tracts of the spinal cord, like the pyramidal 

 tract, and indeed other parts of the brain, are excitable toward 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 576 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 



