RAPIDITY OF NERVOUS CONDUCTION. 103 



noted certain conditions which modify the rate of nervous 

 conduction. One of the most prominent of these, first ob- 

 served by Helrnholtz, is due to modifications in temperature. 

 By a reduction of temperature, in the frog at least, the rate 

 is very much reduced ; and at 32 it is not more than one- 

 tenth as rapid as at 60 or TO . Marey has also noted that 

 the rate is sensibly reduced by fa'tigue of the muscles. 1 



The same principle which has led to the determination of 

 the rate of conduction in motor nerves; viz., an estimation 

 of the difference in time of the passage of a stimulus applied 

 to a nerve at two points situated at a known distance from 

 each other, has been applied to the conduction of sensations. 

 Hirsch is quoted as having made the first attempt id resolve 

 this question, in 1851. a He employed the delicate chrono- 

 metric instruments used in astronomy, and noted the dif- 

 ference in time between the appreciation of an impression 

 made upon a part of the body far removed from the brain, 

 as the toe, and an impression made upon the cheek. This 

 process admitted of the rough estimate of about one hundred 

 and eleven feet per second ; an estimate agreeing remarkably 

 with that of Eaxt for the motor nerves. The later and more 

 elaborate researches of Schelske show a rapidity of conduction 

 by the sensory nerves of about ninety-seven feet per second. 3 



Attempts have been made by Helmholtz, Du Bois-Rey- 

 mond, 4 Marey, 5 Bonders, 6 and others, to estimate the dura- 



1 MAREY, Du mouvement dans lesfonctions de la vie, Paris, 1868, p. 433. 



2 LOXGET, Traite de physiologic, Paris, 1869, tome in., p. 291. 



3 SCHELSKE, Nene Messungen der Fortpflanzungsgeschwindigkeit des Reizes in 

 den menschlichfn Nerven. Archiv fur Anatomic, Physiologic und iribsenchaftUcht 

 M'.'l.rin, Leipzig, 1864, S. 172. 



4 Du BOIS-REYMOND, On the Time required for the Transmission of Volition 

 and Sensation through the Nerves. A Lecture given at. the Royal Institution. 

 BEXCE JOXES, Croonian Lectures on Matter and Force, London, 1868, Appendix 

 L, p. 97, et seq. ; and, Revue des cours scientijiqaes, Paris, 1866-'67, tome iv., p. 

 39, et seq. 



5 MAREY, Du mouvement dans lesfonetions de la vie, Paris, 1868, p. 442. 



6 DOXDERS, Velocity of Cerebral Functions. The Quarterly Journal of Psy- 

 chological Medicine, New York, 1869, vol. iii., p. 763, et seq. 



