330 INNERVATION. [CHAP.XI. 



The impulse of volition, primarily excited in the brain,, acts at the 

 same time upon the gray matter of the cord (its anterior horn) , 

 which in virtue of its association with the former, by means of the 

 fibres of the anterior pyramids, becomes part and parcel of the organ 

 of the will, and therefore as distinctly amenable to acts of the mind as 

 that portion which is contained within the cranium. If we destroy 

 the commissural connexion through the pyramidal fibres, the spinal 

 cord ceases to take part in mental actions ; or, if that connexion be 

 only partially destroyed, that portion of the cord which the injured 

 fibres had associated with the brain is no longer influenced by the 

 mind. Again, if the seat of volition in the brain be diseased, the 

 cord, or part of it, participates in the effects of the disease, as far as 

 regards voluntary actions. That it is not too much to ascribe such 

 power to the pyramidal fibres, appears reasonable, if we consider 

 how the fibres of the corpus callosum, and perhaps other transverse 

 commissures, so connect the hemispheres and other parts of the 

 brain that the separate divisions of a double organ act harmoniously 

 in connexion with the operations of a single mind ; or, that, con- 

 versely, two impressions from one and the same source on a double 

 sentient organ are perceived as single by the mind. 



An objection to this explanation will readily be raised, that the 

 excitation of the anterior horn of the gray matter, in the way 

 stated, does not explain the remarkable power which the will has 

 of limiting its action to one or two, or a particular class of muscles. 

 We reply to this, however, that there can be no reason for denying 

 to the mind the faculty of concentrating its action upon a parti- 

 cular series of the elementary parts of the vesicular matter, or even 

 upon one or more vesicles, if we admit that it can direct its influence 

 to one or more individual fibres, as the advocates of the first and 

 second hypothesis do. If, indeed, we admit the one, we must ad- 

 mit the other ; for whether the primary excitation of a fibre take 

 place in the encephalon or in the spinal cord, the part first affected 

 must probably be (according to our second postulate) one or more 

 vesicles of the gray substance. 



The series of changes which would develop a sensation, admits 

 of the following explanation: A stimulus applied to some part of 

 the trunk or extremities is propagated by the sensitive nerves to 

 the posterior horn of the gray matter of the spinal cord, and from 

 the junction of this part with the brain, either through the direct 

 continuity of the vesicular matter of the cord with that of the centre 

 of sensations, or through longitudinal commissural fibres, analogous 

 to, or even perhaps forming part of, the anterior pyramids, this organ 



