Scientific Lectures. 243 



heat within the body, the actual loss of heat by the production of 

 motion is the equivalent of the 350'' foot-tons which represents 

 external work. This, by a simple calculation, will be found to be 

 250,000 heat units, almost the precise amount by which the heat 

 yielded by the food when burned witliout the body, exceeds that 

 actually evolved by the organism. Moreover, while the total heat 

 given off by the body is 2,500,000 units, the amount of energy 

 evolved as work is equal to about 600,000 heat units ; hence the 

 amount of work done by a muscle is as above stated, one-fifth of the 

 actual energy derivable from the food. One point further. The law 

 of correlation requires that the heat set free when a muscle in con- 

 tracting does work, shall be less than when it efiects nothing ; this 

 fact, too. has been experimentally established by Heidenhain.* 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 correlation 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 tiiis nerve from one extremity to the 

 other. In common language, too, this idea finds currency 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-Keymond has demon- 

 strated by showing that its velocity is only ninety-seven feet in a 

 second, a speed equaled by the greyhound and the race-horse. f In 

 his opinion, the propagation of a nervous impulse is a sort of succes- 

 sive molecular polarization, like magnetism. But that this agent is 

 a force, as analogous to electricity as its magnetism, it 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 contraction of a muscle is excited by 

 diminishing its normal electrical current ;:j; a result which could take 

 place only with a stimulus closely allied to electricity. ISTerve-force, 

 therefore, must be transmuted potential energy. . 



* Heidenhain, op. cit. Also by Fick, Untersuchungen uber Muskel-arbeit, Basel, 1867. Compare 

 also " Xaturc," i, 159, Dec. 9, 1869. 



t Du Bois-Reymond, Emil, On the time required for the transmiesion of volition and sensation 

 through the nerve9, Proc. Roy. Inst. Also in Appendix to Bence J^ones's Crooniau lectures. 



% Marshall, op. cit., p. 227. 



