42 THE STORY OF THE LIVING MACHINE. 



little difficulty in applying natural laws to the ex- 

 planation of living phenomena. But the problem 

 with the nervous system is very different. It is 

 only to-day that we are finding that the problem 

 is open to study, to say nothing of solution. It 

 is true that mental and other nervous phenomena 

 have been studied for a long time, but this study 

 has been simply the study of these phenomena by 

 themselves without a thought of their correlation 

 with other phenomena of nature. It is a matter 

 of quite recent conception that nervous phenom- 

 ena have any direct relation to the other realms of 

 nature. 



Our first question must be whether we can find 

 any correlation between nervous energy and other 

 types of energy. For our purpose it will be con- 

 venient to distinguish between the phenomena of 

 simple nervous transmission and the phenomena 

 of mental activity. The former are the simpler, 

 and offer the greatest hope of solution. If we are 

 to find any correlation between nervous energy 

 and other physical energy, we must do so by find- 

 ing some way of measuring nervous energy and 

 comparing it with the latter. This has been very 

 difficult, for we have no way of measuring a nerv- 

 ous impulse directly. In the larger experiments 

 upon the income and outgo of the body, in the 

 respiration apparatus mentioned above, nervous 

 phenomena apparently leave no trace. So far as 

 experiments have gone as yet, there is no evidence 

 of an expenditure of extra physical energy when 

 the nervous system is in action. This is not sur- 

 prising, however, for this apparatus is entirely too 

 coarse to measure such delicate factors. 



That there is a correlation between nervous 

 energy and physical energy is, however, pretty 



