320 Prof. Helmholtz on the Methods of Measuring 



You see the methods are here for making infinitely finer mea- 

 surements than we need at present. The difficulty now is to 

 apply the method to the special cases, to construct the connect- 

 ing links between the process whose duration is to be determined, 

 and the apparatus to be used for the determination. Indeed, 

 the method must depend upon the object sought. In general I 

 have found Pouillet's electro-magnetic method most advanta- 

 geous, but for certain purposes the rotating cylinder is to be 

 preferred. 



The measurements which I have hitherto made refer partly to 

 the duration of muscular contractions, partly to the velocity with 

 which an impression made upon the nervous fibres is propagated 

 through these fibres. The living muscles in the human and 

 animal body are to be conceived of as strong elastic bands, which, 

 stretched between certain portions of the bony scaffolding, in a 

 tranquil position are either quite lax, or else their tensions com- 

 pletely neutralize each other. The elastic forces of these bands, 

 however, possess the remarkable property that they can be sud- 

 denly changed by the influence of the nerves. The state thus 

 brought about by the operation of the nerves is called the state 

 of muscular activity. The active muscle behaves also as an 

 elastic band, but it strives to shorten itself with far greater force 

 than the inactive one. The consequence of this change in the 

 living body is, that the force of the active muscle overpowers 

 that of the inactive, the equilibrium of the members is destroyed, 

 and the points at which the muscle is attached to the bones are 

 caused to approach each other. In the living body the muscle 

 receives the excitation to activity from the threads of nerves 

 which ramify through it ; these, in their turn, from the brain. 

 Here the mysterious influence of the will imparts an excitation 

 whose nature is unknown, which propagates itself throughout 

 the entire length of the fibres, and arriving at the muscle excites 

 it to action. If we modernise the comparison of Menenius 

 Agrippa, who pacified the starving plebeians by wisely likening 

 the state to the human body, then the nervous fibres might be 

 compared with the wires of the electric telegraph, which in an 

 instant transmit intelligence from the extremities of the land to 

 the governing centre, and then in like manner communicate the 

 will of the ruling power to every distinct portion of the land. 

 The principal question which I have sought to answer is the 

 following : — In the transmission of such intelligence, is a mea- 

 surable time necessary for the ends of the nerves to communicate 

 to the brain the impression made upon them ; and on the other 

 hand, is time required for the conveyance of the commands of 

 the will from the brain to a distant muscle ? 



This, perhaps, appears improbable ; in our own case we have 



