THE CIRCULATION OF THE BLOOD. 445 



ledged, that these agents were ministerial to higher 

 agency, more difficult to trace than these, but more 

 truly the cause of the phenomena. 



The discovery of the mechanical and chemical 

 conditions of the vital functions, as a step in phy- 

 siology, may be compared to the discovery of the 

 laws of phenomena in the heavens by Kepler and 

 his predecessors, while the discovery of the force 

 by which they were produced was still reserved 

 in mystery for Newton to bring to light. The sub- 

 ordinate relation of the facts, their dependence on 

 space and time, their reduction to order and cycle, 

 had been fully performed; but the reference of 

 them to distinct ideas of causation, their interpreta- 

 tion as the results of mechanical force, was omitted 

 or attempted in vain. The very notion of such 

 force, and of the manner in which motions were 

 determined by it, was in the highest degree vague 

 and vacillating ; and a century was requisite, as we 

 have seen, to give to the notion that clearness and 

 fixity which made the mechanics of the heavens a 

 possible science. In like manner, the notion of life, 

 and of vital forces, is still too obscure to be steadily 

 held. We cannot connect it distinctly with severe 

 inductions from facts. We can trace the motions of 

 the animal fluids, as Kepler traced the motions of 

 the planets ; but when we seek to render a reason 

 for these motions, like him we recur to terms of a 

 wide and profound, but mysterious import ; to vir- 

 tues, influences, undefined powers. Yet we are not, 



