108 



UN THE PRINCIPLE OF LIFEj 



motions, and thus constitutes a fourth class ; which are hence named as- 

 sociate stimuli^ or stimuli of association. 



But though the muscular fibre is, perhaps, more irritable than any other 

 part of the system, the principle of irritability and a fibrous structure are 

 by no means necessarily connected ; for, while the cellular membrane m 

 fibrous but has no irritability whatever, the skin is not fibrous but is highly 

 irritable. 



Hence solids and fluids are equally necessary to the perfection of the 

 living system. Food, air, and the ethereal gases, caloric, oxygene, an^ 

 the medium of electricity, are the stimuli by which it is chiefly excited to 

 action ; and, by their combination, contribute in some degree to the matter 

 of the system itself; but of the mysterious power that developes the organs 

 and applies the stimuli, that harmonizes the action and constitutes the hfe, 

 we know nothing. 



We see clearly, however, that the moving powers are, for the most part, 

 the muscles ; and it is a subject of perpetual astonishment to the physiolo- 

 gist to observe the prodigious force which these vital chords are made ca- 

 pable of exerting, and the infinite variety of purposes to which they thus 

 become subservient. And were it not that the whole universe swarms 

 with proofs of intelligence and design— were it not that there exists, to 

 adopt the beautiful words of the poet — 



Books in the running brooks, 

 Sermons in stones, and good in every thing— 



4his, perhaps, might be the part of creation which we could best select in 

 proof of the wisdom of the Creator. 



It was formerly too much the custom to regard the animal frame as a 

 mere mechanical machine ; whence, in that spirit of absurdity with which 

 the wisest of mankind are occasionally afilicted, Descartes affected to be- 

 lieve that brutes are as destitute of consciousness as a block of wood, and 

 that it is exactly the same sort of necessity which drives a dog forward in 

 pursuit of a hare, that compels the different pipes of an organ to give forth 

 different tones upon a pressure of the fingers against its diflferent keys. It 

 is not every one, however, in modern times who has adopted the mecha- 

 nical theory that has carried it to this extremity of absurdity ; but all of 

 them are still carrying it too far who reason concerning the principal mo- 

 tions of the body as mere mechanical motions, and the powers which the 

 muscles exert as mere mechanical powers ; in which the bones are the 

 levers, the joints the fulcra, and the nmscles the moving cords ; for it so 

 happens that all the effects for which the whole of this complicated ma(- 

 chinery is absolutely necessary out of the body, are in many instances per- 

 formed by a single part of it within the body, namely, by the moving cords 

 or muscles alone, without either bones or joints, levers or fulcra. I do not 

 mean to contend that there is no kind of resemblance or conformity of prin- 

 ciple between the laws of animate and inanimate mechanics, for I well 

 know that in a variety of points the two systems very closely concur : but 

 1 am obliged to contend th^t they are still two distinct systems, and that in 

 the one case the living power exercises an influence which finds no sort of 

 similitude in the other. 



It is indeed curious to observe the difference of result which has flowed 

 from the calculations of the different promoters of this theory ; and which 

 alone, were there nothing else to oppose them, would be sufficient to prove 



