296 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



must add, however, the provision that the two things 

 are aperiodic. If they be periodic, the opposite is 

 true. We cannot solve an equation while jumping 

 over a broad ditch. According to Fechner's inter- 

 pretation of this fact the brain at any time has only a 

 certain amount of energy at its disposal. In jump- 

 ing over a ditch all the energy is supposed to pass 

 into the muscles and nothing is left for the process 

 of thinking. I showed fourteen years ago that Fech- 

 ner's conception was not correct. The inhibition of 

 a process of thought by simultaneous muscular ac- 

 tivity is greater when we innervate one arm than 

 when we innervate both arms simultaneously. Ac- 

 cording to Fechner, however, the greater the number 

 of muscle-groups that were innervated the more 

 energy must be consumed in the brain. In these ex- 

 periments I measured the maximal pressure which 

 the flexors of the hand are able to exert on a dynamo- 

 meter. This pressure does not decrease when the 

 other hand or all the muscles are innervated simul- 

 taneously, but even increases (3). The further appli- 

 cation of this method explained the fact that we 

 cannot well be mentally and physically active at the 

 same time. 



If we begin by solving a moderately difficult prob- 

 lem in mental number work, and if we attempt when 

 in the midst of the task to attain the highest dynamo- 

 metrical pressure with the hand, the pressure re- 

 mains from 20-30^ below the maximum that we 

 otherwise attain when we devote our attention to the 



