CHAP, ii.] THE BEAIN. 809 



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 4 currents of action ' referred to in 486. We 

 are already going beyond our tether when we assume on the 

 strength of this that the processes started in the fibres of the 

 pyramidal tract by artificial stimulation are in all respects identi- 

 cal with those started in the fibres of a motor nerve. We are 

 going still more beyond our tether when we assume that the 

 processes started in the same pyramidal fibres as the outcome of 

 natural events in the motor cortex are of the same kind. But 

 these assumptions are trifles compared with the assumption that 

 the events taking place in the fibres of the optic radiation, pass- 

 ing from the pulvinar to the occipital cortex are identical with 

 the events taking place in the fibres of the optic tract on the way 

 to the pulvinar, or that the events travelling along the spinal 

 cord to the brain as the result of a prick of the little finger are 

 identical with those which the prick has started in the fibres 

 of the ulnar nerve. Of the latter events we know a little ; of 

 the former events we know next to nothing. And we may here 

 ask the question what is the meaning of these continual relays 

 of grey matter along the sensory tract unless it be that at each 

 relay, some transformation, some further elaboration of the 

 impulses takes place, until what were the relatively, but only 

 relatively, simple impulses along the fibres of the peripheral 

 nerve are by successive steps changed in the complex events 

 which we call a conscious sensation? We have no reason to 

 think that the afferent impulses started at the periphery of a 

 cutaneous nerve-fibre change essentially in character as they 

 travel over the fibre along the stretch of nerve of which it is a 

 part. Nor have we any satisfactory evidence that any change in 

 the character of the impulses is effected by the nerve cell in the 

 root-ganglion with which the fibre is connected by means of a 

 T piece, though this is possible. Within the cord things are 

 different. The arborescent ending of the fibre of the posterior 

 root within the spinal cord (or bulb) is in contact with, not in 

 continuity with, the cell-substance of the cell, or trie processes of 

 the cell on which it impinges. Of course it is possible that the 

 ending should set up in that cell-substance molecular changes 

 identical with those which constitute the impulse passing along 

 the fibre ; but it is much more probable that the changes which 

 it sets up are of a different order, the transition being in a rough 

 way comparable to the transition from a nervous impulse reach- 



