HERMANN VON HELMHOLTZ 



determined, then this distance represented a certain 

 period of time. 



By this method a single experiment gave results 

 that could only have been obtained, with much 

 trouble and possibilities of inaccuracy, by a whole 

 series of observations by Pouillet's method. The ex- 

 periments showed that in the motor nerves of the 

 frog the velocity of the nervous impulse was only 

 about ninety feet per second (that of a very quick 

 express train), or about T Vth part of the velocity of 

 sound in air, a result quite unexpected. Until the 

 question was submitted by Helmholtz to the test of 

 experiment, the wildest conjectures had been indulged 

 in by speculative physiologists. Thus the iatro- 

 mathematicians of Montpellier said that the rapidity 

 bore a ratio to that of the blood in the aorta, namely, 

 in the proportion of the diameter of the aorta to that 

 of a nerve fibre, a statement that implied a velocity 

 of the nervous impulse 600 times more rapid than 

 light ! Haller took as the basis of his conjectures 

 the number of vibrations made by the tongue in pro- 

 nouncing the letter R, and, by a series of deductions, 

 most of them wide of the mark,' he strangely arrived 

 at a conclusion not very far off what we know to 

 be correct, namely, that the velocity was about 150 

 feet per second. 1 Johannes Miiller despaired of being 



1 Haller's Elementa, t. iv., p, 372 ' Ego vero, cum haec mere theoretica 

 sint experimento uti malim, etsi minor summa prodit. Ita invenio 

 summam tamen celeritatem esse muscularis liquid! ut non minus quam 

 9000 pedes in minuto percurrat.' 



6 4 



