iv GENERAL PHYSIOLOGY OF NERVOUS SYSTEM 203 



From the difference in time and the length of the nerve as 

 measured the rate of conductivity is easily calculated. 



Helmholtz employed two methods for determining tlie time that elapses 

 between the (electrical) stimulation of the nerve and the reaction of the muscle. 

 The first method, invented by Pouillet, consists in measuring the duration 

 of an electrical current, sent through a galvanometer at the moment of excit- 

 ing the nerve, and interrupted at the moment at which the muscle contracts 

 (for details of the application of this method see Biedermann). 1 The second 

 method, employed after Helmholtz by all physiologists, is a special applica- 

 tion of the graphic method. The times of nerve excitation and muscle con- 

 traction are recorded by a myograph on the smoked paper of a drum or plate, 

 which is moving very rapidly, the time being marked on the same surface 

 by means of a tuning-fork. The difference in time can thus be measured 

 exactly between the first stimulation of the nerve close to the muscle and the 

 commencement of the muscular contraction, and the second stimulation 

 farther from the muscle and commencement of the second; contraction. 

 When the times of the two successive stimulations are recorded at the same 

 point of the revolving drum (as in Fig. 133), the distance between the initial 



Fio. 133. Velocity of nerve conduction, as measured by Marey on himself. 1, rnyogram traced on 

 exciting the nerve close to the muscle ; 2, myogram on exciting the nerve 30 cms. from the 

 muscle ; D, time tracing from a tuning-fork at 250 double vibrations per second. The interval 

 between the two contractions occupies about 2'5 vibrations, corresponding to O'Ol sec. in 

 which the impulse traverses 30 cms. = 30 m. per second. 



point of the two contractions is all that is required to calculate the rate of 

 conductivity, when the length of nerve between the two points of excitation 

 is known. 



From an average of the experiments made by Helmholtz on 

 the frog's nerves the velocity of nerve conduction was found to be 

 2 7 '25 metres per second, which is much less than the velocity of 

 the propagation of sound in air, but greater than the propagation 

 of the contraction wave in the muscle of the same animal, this 

 being, as we have seen, about 1 metre per second. 



Helmholtz and Baxt also determined the rate of conductivity 

 in the motor nerves of man. They recorded the myograms of the 

 thumb-muscle upon a rotating cylinder by placing a sensitive 

 lever on the thenar eminence, and exciting the median nerve 

 either in the axilla or near the wrist joint, through the previously 

 moistened skin. The rate obtained was somewhat higher than for 

 frog nerves, i.e. 30-35 m. per second. 



Helmholtz and many other investigators have also attempted 

 to determine the rate at which the impulse is propagated in the 



1 Electro- Physiology, English translation by F. A. Welby, 1896, ii. 59. 



