1903.] on the Paths of Volition. 277 



can only be originated in the brain in consequence of a sensation being 

 conducted to the brain from the part which is to be moved, a sensation 

 probably emanating from the muscles of the part themselves; but it 

 appears to me that the true explanation of the experiment is that 

 which has been given by Dr. Charlton Bastian, who has pointed out 

 that the severance of the posterior roots, by cutting off all afferent 

 impressions from the limb to the grey matter of the cord, will abolish 

 that condition of the muscles of the limb which is known as muscular 

 tonus, by virtue of which the muscles are always kept in a condition 

 of readiness to contract, and without which far stronger impulses 

 from the brain are required to act upon the motor nerve cells of the 

 cord in order to induce movements of the muscles. Mott and 

 Sherrington indeed noticed that when an animal was executing 

 movements in an energetic fashion, the paralysed arm participated 

 in the movements, and they also found that the apparently paralysed 

 muscles can be called into activity by electrical excitation of the 

 corresponding part of the cerebral cortex. 



If vve apply the teaching of this experiment to the experiment 

 which I have just placed before you — viz., that section of the antero- 

 lateral columns of the cord produces paralysis of voluntary motion 

 below the lesion — we see that it is possible to explain the paralysis 

 not necessarily by the supposition that we have severed a volitional 

 path alon<i the antero-lateral tract, but by supposing that the severance 

 of these fibres of the antero-lateral tract has cut off impressions which 

 were proceeding to the anterior horn cells ; impressions which were 

 helping to keep them in such a condition of tonus as would render 

 them pre[)ared to send out nervous impulses with great facility along 

 the corresponding motor nerves. That the section of the antero-lateral 

 tracts may produce paralysis as a consequence of such loss of tone, 

 is rendered even more probable by an observation ujade by Professor 

 Ewald, who found that if that part of the auditory nerve — the vesti- 

 bular branch — which comes from the semicircular canals is severed, 

 or if the semicircular canals themselves are destroyed, there is pro- 

 duced, besides other symptoms, diminution or loss of tone in the 

 muscles of the body generally. Now many of the fibres of the vesti- 

 bular nerve pass to the nucleus of Deiters, from which, as we have 

 seen, the fibres of the posterior longitudinal bundle, which go to the 

 cord, originate ; and it is therefore very possible that these fibres may 

 carry the impressions derived from the vestibular nerve which help 

 to maintain the tone of the motor cells of the cord. Of course the 

 cutting of the antero-lateral descending tracts would cut off such 

 impressions, and the tone of the motor-cells would be thereby 

 diminished. Further, it must be borne in mind, that the antero- 

 lateral descending tracts of the cord also include the fibres of the ven- 

 tral longitudinal bundle coming from the mid-brain, and descending 

 in all probability from cells with which the tibres of the optic nerve 

 are in close connection. And it is probable that impulses arriving 

 along this sensory path also assist in maintaining the tone of the 



