444 HISTORY OF PHYSIOLOGY. 



systole and diastole, caused the circulation of the 

 blood, it might still be asked, what force caused 

 this constantly -recurring contraction and expan- 

 sion. And again, circulation is closely connected 

 with respiration; the blood is, by the circulation, 

 carried to the lungs, and is there, according to the 

 expression of Columbus and Harvey, mixed with 

 air. But by what mechanism does this mixture 

 take place, and what is the real nature of it ? And 

 when succeeding researches had enabled physiolo- 

 gists to give an answer to this question, as far as 

 chemical relations go, and to say that the change 

 consists in the abstraction of the carbon from the 

 blood by means of the oxygen of the atmosphere ; 

 they were still only led to ask further, how this 

 chemical change was effected, and how such a 

 change of the blood fitted it for its uses. Every 

 function of which we explain the course, the me- 

 chanism, or the chemistry, is connected with other 

 functions, is subservient to them, and they to it; 

 and all together are parts of the general vital sys- 

 tem of the animal, ministering to its life, but 

 deriving their activity from the life. Life is not a 

 collection of forces, or polarities, or affinities, such 

 as any of the physical or chemical sciences contem- 

 plate ; it has powers of its own, which often super- 

 sede those subordinate relations; and in the cases 

 where men have traced such agents in the animal 

 frame, they have always seen, and usually acknow- 



