202 INNERVATION. [CHAP. vm. 



In every ordinary voluntary action, the first step is a mental 

 change, in which consists the act of volition. The mind is per- 

 fectly able to induce this change in itself, without any reference to 

 the body ; but if it direct its influence upon certain muscles, the 

 contraction of those muscles immediately ensues, in a combined 

 and regular manner, so as to produce the predetermined voluntary 

 action. But the influence of the mind cannot be brought to bear 

 upon the muscles, save through the intervention of the nerves, as is 

 amply proved by the destruction of certain voluntary movements, 

 which is consequent upon the destruction of certain nerves. 



Again, in all cases of common or of special sensation, that state 

 of the mind, in which the sensation consists, is induced by an im- 

 pression made upon certain bodily organs, and conveyed to the 

 mind through the instrumentality of the nerves. For there is 

 abundant evidence to prove, that, while the mind is of itself capable 

 of entering that state, it cannot do so in obedience to bodily change, 

 if certain nerves be destroyed or impaired; that, in short, the 

 nerves are the only corporeal channel through which sensations 

 can be excited. If the skin be forcibly irritated or compressed, 

 instantly pain is felt ; but, were the nerves of the skin destroyed, 

 no degree of irritation or pressure would make the mind cognizant 

 of the injury. Light is admitted to the eye, and forthwith a cor- 

 responding affection of the mind ensues ; but, for the production 

 of this, the integrity of the optic nerve is a necessary condition. 



In these examples of nervous action, it will be observed that, in 

 the former instance, mental change produces bodily action ; and, 

 in the latter, an impression upon some part of the body precedes 

 and gives rise to an affection of the mind. In both cases nervous 

 power is called forth : in the one, it acts in the direction from mind 

 to body ; in the other, from body to mind. In both cases, destruc- 

 tion of the nervous matter would prevent the development of the 

 force. The muscles may be sound, and the will may be vigorous ; 

 but without perfect nerves the latter cannot impart its mandates to 

 the former. Or the eye may be perfect in all its optical adjust- 

 ments, and the mental sensibilities keen and quick; and yet, if 

 the optic nerve be diseased, the light which falls upon the retina 

 produces no impression upon the mind. 



" Of the nature of the connexion of this great sensorial organ" 

 (the nervous system), says Dr. Brown, "with the sentient mind, 

 we never shall be able to understand more than is involved in the 

 simple fact, that a certain affection of the nervous system precedes 

 immediately a certain affection of the mind. But though we are 



