GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 91 



estimated by the effect produced on the organ which the nerve excites to action, 

 or by the change which takes place in the electrical condition of the nerve as 

 the wave of excitation sweeps over it. 



Rate in Motor Nerves. Helmholtz was the first to measure the rate of con- 

 duction in nerves. 1 Originally he employed Pouillet's method for measuring 

 short intervals of time. The arrangement is illustrated in Figure 32. The 

 moment that the current in the primary coil of an induction apparatus was 

 broken and the nerve connected with the secondary coil received a shock, 

 a current was thrown into the coils of a galvanometer (see p. 1 36). An instant 

 after, the contraction of the muscle which resulted from the stimulation of the 

 nerve broke the galvanometer circuit. The amount of deviation of the magnet 

 of the galvanometer varied with the time that the circuit remained closed, and 

 therefore could be taken as a measure of the interval elapsing between the 

 stimulation of the nerve and the contraction of the muscle. The nerve was 

 excited in two succeeding experiments at two points, at a known distance apart, 

 and the difference in the time records obtained was the time required for the 

 transmission of the nerve-impulse through this distance. 



FIG. 32. Method of estimating rate of conduction in motor nerve of frog, as used by Helmholtz. 

 The horizontal bar a-6 is supported on an axis in such a manner that when the contact is made at a it is 

 broken at b, therefore at the same instant a current is made in the galvanometer circuit and opened in 

 the primary circuit of the induction apparatus. When the muscle contracts, the galvanometer circuit 

 is broken at c. The nerve was stimulated in two successive experiments at d and e. 



Later, Helmholtz devised a method of directly recording the contractions 

 of the muscle, and employed this to measure the rate of conduction in motor 

 nerves. He stimulated the nerve as near as possible to the muscle and re- 

 corded the contraction, then he stimulated the nerve as far as possible from the 

 muscle and again recorded the contraction. The difference in time between 

 the moment of excitation and the beginning of the contraction in the two 

 experiments was due to the difference in the distance that the nerve-impulse 

 had to pass in the two cases, and, this distance being known, the rate of con- 

 duction could be readily calculated. By this means he found the rate of trans- 

 mission in the motor nerves of the frog to be 27 meters per second. In 

 similar experiments upon men he recorded the contractions of the muscles of 

 the ball of the thumb, and noted the difference in the time of the beginning 

 of the contractions when the median nerve was excited through the skin at two 

 1 Helmholtz : Arehiv fur Anatomie und Physiologic, 1850, p. 71-276 ; 1852, p. 199. 



