IS THE BODY A MACHINE? 55 



phenomena of mental activity. The former are 

 the simpler, and offer the greatest hope of 

 solution. If we are to find any correlation be- 

 tween nervous energy and other physical energy, 

 we must do so by finding 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 nervous 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 surprising, 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 

 definitely proved by experiments along different 

 lines. The first step in this direction was to find 

 that a nervous stimulus can be measured at least 

 indirectly. When the nerve is stimulated there 

 passes from one end to the other an impulse, 

 and the rapidity with which it travels can be 

 accurately measured. When such an impulse 

 reaches the brain it may give rise to a con- 

 scious sensation, and a somewhat definite esti- 

 mation can be made of the amount of time 

 required for this. The periods are very short, 

 of course, but they are not instantaneous. The 

 nervous impulse, can be studied in still other 

 ways. We find that the impulse can be started 

 by ordinary forms of energy. A mechanical 



