FUNCTIONS OF THE SPINAL CORD. 137 



reason for maintaining that sensation is an element in their production, whilst 

 it is admitted to be not essential in the case of the less regular convulsive 

 actions already described. The character of adaptiveness to a designed end, 

 in regard to their combination and succession, which the movements of respira- 

 tion and deglutition exhibit, is clearly no proof of their dependence on sensa- 

 tion ; since an equally perfect adaptiveness is witnessed in the actions of 

 the heart, alimentary canal, &c., which are still further removed from the 

 control of the will. And, further, it does not appear by any means evident, 

 what end or purpose could be answered by the production of sensation, as a 

 part of the chain of phenomena of reflex action. The question is, are these 

 movements guided in any way by the mind ; or do they necessarily result 

 from certain physical conditions of the nervous system ? If their adaptive- 

 ness is the result of mental guidance, then not only sensation, but an operation 

 either of instinct or of intelligent will must necessarily be involved ; since it 

 is impossible that sensation can guide to the choice of one out of many modes of 

 action, without the exercise of these faculties. On the other hand, if it be 

 said that certain movements are from t'he first necessarily associated with cer- 

 tain sensations, it is difficult to see why they should not be equally associated 

 with the impressions by which the sensations are produced. Sensation is a 

 psychical phenomenon. It is the communication to the mind, of a certain 

 organic change in the nervous system. It is the first step in the train of 

 purely mental operations; and these terminate in the formation of an instinc- 

 tive or volitional impulse, which reacts on the body. But we have no reason 

 to believe that sensation can itself react on the body ; or that, if it could, it 

 would be a better guide than the impression which produced it. Thus 



Impression a produces 



Sensation A, with which is associated 



Motion a ; 

 and in like manner, 



Impression b produces 



Sensation B, with which is associated 



Motion )3. 



There seems no valid reason, then, to assert that a motign may not have an 

 equally close connection with the impression, as it is asserted to have with 

 the sensation resulting from it. 



181. The question has been often put to those who advocate this view 

 why the sensation should be so constantly associated with these changes, if 

 not essential to produce the motion? An objection might fairly be made to 

 any reasoning from final causes, in a question of facts ; but the inquiry may 

 be easily answered. In many instances the production of sensations is the 

 stimulus necessary for the excitement of other actions, which are required for 

 the continued maintenance of those in question. This may be rendered more 

 comprehensible by a simple illustration. A cistern filled with water may be 

 speedily emptied by a cock occasionally opened at the bottom ; but, if it com- 

 municate with a reservoir, by means of a valve opened by a ball floating on 

 the surface of the water it contains, it may be kept constantly full. The lower 

 cock is opened, and the water flows out ; and, in consequence of the lowering 

 of the surface thus produced, the floating valve above is opened, and the cistern 

 is refilled from the reservoir. Now here the action of the ball-cock at the top is 

 not essential to the flow of water at the bottom, but is rather consecutive upon 

 it. Just so it is with regard to those movements of Animals, which are con- 

 cerned in the ingestion of their food. The muscular contractions required to 

 propel it along the alimentary canal, from the stomach downwards, are pro- 

 vided for, without even the intervention of the nervous system. To bring it 



12* 



