598 -NERVOUS SYSTEM. 



The important and interesting results of these experiments were arrived at by an inge- 

 nious application of the graphic method, which has since been so largely improved and 

 extended by Marey, and their accuracy was rendered possible by the exceedingly delicate 

 chronometric apparatus which has been devised within the last few years. 



It is unnecessary to describe fully the exact methods employed by Helmholtz and 

 by those who immediately followed in his investigations. Suffice it to say, that this dis- 

 tinguished physiologist and physicist constructed apparatus which, though somewhat 

 complex, was so accurate as to leave no doubt as to the reliability of his results. Taking 

 into account all of the disturbing conditions, and allowing for the interval of pose, or the 

 length of time between the excitation of a muscle and the commencement of its con- 

 traction, he estimated the rapidity of conduction in the motor nerves of the frog at about 

 eighty-five feet per second. The results obtained by Marey upon frogs give a much 

 slower rate of nervous conduction. These were followed, however, by the observations 

 of Helmholtz and Baxt upon the human subject, which are, of course, the most interest- 

 ing of all. 



The process devised by Marey is admirably simple. He employed, to estimate small 

 fractions of a second, a cylinder graduated in the following manner : An ordinary tun- 

 ing-fork, vibrating, say, five hundred times per second, is so arranged that a point con- 

 nected with one of its arms is made to play against a strip of blackened paper. As the 

 paper remains stationary, the point makes but a single mark ; but when the paper 

 moves, as the point vibrates, a line is produced with regular curves, every curve repre- 

 senting yi^ of a second. Now, if a lever be attached to a muscle and be so arranged as 

 to mark upon the paper, moving at the same rate, the instant when contraction takes 

 place, it is evident that the interval between two contractions produced by stimulating 

 the nerve at different points in its course will be most accurately indicated ; and, if the 

 length of the nerve between the two points of stimulation be known, the difference in 

 time will represent the rate of nervous conduction. 



In experiments upon frogs, the leg is prepared by cutting away the muscles and bone 

 of the thigh, leaving the nerve attached. The lever is then applied to the muscles of 

 the leg, and the stimulation is applied successively to two points in the nerve, the distance 

 between them being carefully measured. The results obtained in this way showed a 

 rate of conduction of from thirty-six to forty-six feet per second ; but these are not 

 regarded by Marey as invalidating the estimates by Helmholtz, in view of the various 

 conditions by which the rapidity of conduction'is modified. 



Employing the myograph of Marey, Baxt, in the laboratory of Helmholtz, has suc- 

 ceeded in measuring the rate of nervous conduction in the human subject. In these 

 experiments, the swelling of the muscle during contraction was limited by enclosing the 

 arm in a plaster-mould and noting the contraction through a small opening. By then 

 exciting the contraction by stimulating the radial nerve successively at different distances 

 from the muscle, the estimate was made. The rate in the human subject was thus esti- 

 mated at one hundred arid eleven feet per second. The latest experiments upon this sub- 

 ject by Helmholtz and Baxt, in which great care was taken in the adjustment of the 

 apparatus, showed a mean of rapidity for the motor nerves, in man, of about two hun- 

 dred and fifty-four feet per second. These observations were made in the summer of 

 1869 ; and the difference in the results is in part explained by the fact, which was ascer- 

 tained experimentally at that time, that a high temperature increases, and a diminished 

 temperature retards the velocity of nervous conduction. It has been farther shown by 

 Munk, that the rate of conduction is different in different portions of the nervous trunk ; 

 the rapidity progressively increasing as the nerve approaches its termination. 



Helmholtz, Du Bois-Reymond, Marey, and others, have noted certain conditions 

 which modify the rate of nervous conduction. One of the most prominent of these, 

 first observed by Helmholtz, 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 



