460 LECTURE XIX. 



should nevertheless revive the excitability as shown by the oxidation and 

 development of energy. It is not necessary in every case that all of the 

 hydrolytic products should be consumed. The cell is able to be very eco- 

 nomical with its fuel. On the other hand, it is perfectly possible that 

 oxygen under these conditions may play an unusual part in the mechanism 

 of the cell work. We cannot work out such problems successfully until we 

 have a more accurate insight into the oxidations of the cells and tissues, 

 and as long as our physical and chemical conceptions of the proto- 

 plasm are still vague. We meet here with a great many riddles, which 

 for the present are unanswerable for the present, we say, because 

 we can scarcely doubt that before long the rapid progress of biological 

 chemistry will bring clearness to our conception of these complicated 

 processes. An advance in the science is only possible, however, when it 

 is clearly and sharply recognized where the facts end and the hypotheses 

 begin. We can only build upon the former, and the latter serve merely 

 as a framework which is of value only when it is possible to replace the 

 fantasies of the brain little by little with facts verified by experimentation. 



