472 HISTORY OF PHYSIOLOGY. 



spirit is not than what it is, he conceives that, while 

 it must be far too fine to be perceived by the sense, 

 it must yet be more gross than fire, magnetism, or 

 electricity ; so that it may be contained in vessels, 

 and confined by boundaries. And Cuvier speaks to 

 the same effect 17 : " There is a great probability that 

 it is by an imponderable fluid that the nerve acts 

 on the fibre, and that this nervous fluid is drawn 

 from the blood, and secreted by the medullary 

 matter." 



Without presuming to dissent from such autho- 

 rities on a point of anatomical probability, we may 

 venture to observe, that these hypotheses do not 

 tend at all to elucidate the physiological principle 

 which is here involved ; for this principle cannot be 

 mechanical, chemical, or physical, and therefore 

 cannot be better understood by embodying it in a 

 fluid ; the difficulty we have in conceiving what the 

 moving force is, is not got rid of by explaining the 

 machinery by which it is merely transferred. In 

 tracing the phenomena of sensation and volition to 

 their cause, it is clear that we must call in some 

 peculiar and hyperphysical principle. The hypo- 

 thesis of a fluid is not made more satisfactory by 

 attenuating the fluid ; it becomes subtle, spirituous, 

 ethereal, imponderable, to no purpose; it must 

 cease to be a fluid, before its motions can become 

 sensation and volition. This, indeed, is acknow- 

 ledged by most physiologists; and strongly stated 



17 Regne Animal, Introd. p. 30. 



