56 ( 20 ; 



experimentally established by Heidenhain. 2 ? So, again, 

 when muscular contraction does not result in motion, 

 as when one tries to raise a weight too heavy for him, 

 the energy which would have appeared as work, takes 

 the form of heat : a result deducible by the law of cor- 

 relation from the steam-engine. 



The last of the so-called vital forces which we are to 

 examine, is that produced by the nerves and nervous 

 centers. In the nerve which stimulates a muscle to 

 contract, this force is undeniably motion, since it is 

 propagated along this nerve from one extremity to the 

 other. In common language, too, this idea finds cur- 

 rency in the comparison of this force to electricity ; the 

 gray or cellular matter being the battery, the white or 

 fibrous matter the conductors. That this force is not 

 electricity, however, Du Bois-Reymond has demonstrated 

 by showing that its velocity is only 97 feet in a second, 

 a speed equaled by the greyhound and the race-horse. 3 

 In his opinion, the propagation of a nervous impulse is 

 a sort of successive molecular polarization, like mag- 

 netism. But that this agent is a force, as analogous to 

 electricity as is magnetism, is shown not only by the 

 fact that the transmission of electricity along a nerve 

 will cause the contraction of the muscle to which it 

 leads, but also by the more important fact that the con- 

 traction of a muscle is excited by diminishing its nor- 

 mal electrical current ;3* a result which could take place 

 only with a stimulus closely allied to electricity. Nerve- 

 force, therefore, must be a transmuted potential energy. 



What, now, shall we say of that highest manifestation 

 of animal life, thought-power ? Has the upper region 

 called intelligence and reason, any relations to physical 



