CHAP. XI.] THEOKY OF NERVOUS ACTIONS. 331 



is simultaneous affected. To this likewise it will be objected, that 

 the limitation of sensations is not sufficiently explained. But the 

 reply is obvious : the intensity and kind of sensation depend upon 

 the nature of the primary stimulus at the surface ; the extent upon 

 the number of fibres there stimulated. Wherever these fibres form 

 their proper organic connection with the vesicular matter, that matter 

 will participate in their change to an extent proportionate to the 

 number of fibres stimulated, and with an intensity commensurate 

 with the force of the primary stimulus. It is not necessary to the 

 development of sensation that the fibre stimulated should be im- 

 planted directly in the brain ; if it be connected with this centre 

 through the medium of vesicular matter of the same character as 

 that which is found in it or through commissural fibres, all condi- 

 tions necessary for the development and propagation of nervous 

 force would appear to be fulfilled. It must not be supposed, how- 

 ever, that in making this statement we mean to assign the spinal 

 cord to be the seat of sensation ; all we assert is, that the posterior 

 horns of its gray matter, as being the part in which the sensitive 

 roots are implanted, participate largely in the mechanism of sensa- 

 tion ; and that by their union with the brain they become, pro tanto, 

 a part of the centre of sensation, so long as that union is unimpaired. 



This hypothesis offers an explanation of the hitherto unexplained 

 phenomenon of impaired sensation on that side of the body which 

 is opposite to the seat of cerebral lesion. If we regard the anterior 

 pyramids as commissures between the sensitive, as well as between 

 the motor portions of the cerebro-spinal centre, it will be obvious 

 that the posterior horns of the spinal gray matter on the right side 

 will be associated with the left centre of sensation in the brain, and 

 vice versa. 



And we gain, moreover, an explanation of the almost universal 

 association of sensation with reflex or physical actions. The excitor 

 nerves of these actions being the same as the sensitive nerves, the 

 impression conveyed by them is calculated at once to excite motion 

 and sensation. Were it not for the controlling influence of the 

 will, all sensitive impressions made through the spinal cord would 

 likewise be accompanied by corresponding movements. When the 

 spinal cord has been excited by strychnine, the physical power pre- 

 vails over the mental, and the will ceases to be able to control the 

 movements excited by impressions through sensitive nerves. 



A highly important argument in favour of this view is derived 

 from the marked difference of structure of the anterior and posterior 

 horns of the spinal vesicular matter. The anterior and posterior 



