ADVANCED EXPERIMENTAL PHYSIOLOGY 113 



CHAPTER IX 

 THE RATE OF TRANSMISSION OF A NERVOUS IMPULSE 



The rate at which an impulse is transmitted along a nerve is 

 important because it throws some light upon the nature of the 

 impulse. It travels much more slowly than an ordinary electric 

 current, and, although it is accompanied by an electric change, it is 

 something more complex. Its rate of propagation is 27 metres per 

 second (88 J feet per sec.) in the frog's sciatic nerve, and 120 metres 

 per second in the motor nerves of man. 



(a) In the Motor Nerves of the Frog. See Part I, p. 53. 



(6) In the Motor Nerves of Man. The velocity of the trans- 

 mission of a nervous impulse in the motor nerves of man can be 

 determined in the following way : A thick-walled india-rubber ball, 

 similar to that used with a photographic " shutter," is connected 

 with a recording tambour. Two clinical electrodes are moistened 

 with strong saline solution in order to improve their conduction and 

 contact with the skin ; the large flat electrode is fastened to the leg 

 of the subject, and the small electrode placed above the clavicle 

 will be pressed over the brachial nerves. These electrodes are 

 connected with the secondary coil of an inductorium, and in the 

 primary circuit is interposed the " striker " key. 



The india-rubber ball is held between the middle finger and the 

 thumb, and the contraction of the flexor muscles will be recorded by 

 the lever of the tambour, when the nerve is excited. The moment of 

 stimulation is determined in the usual way (p. 42), and then the 

 experiment is again performed, but with the small electrode pressed 

 over the median nerve at the bend of the elbow. The moment of 

 stimulation is again determined, in order to show that the resting 

 position of the point of the lever has not been changed. The 

 difference between the latency in the two contractions is measured 

 by a tuning fork vibrating 100 times per second, and the length of 

 nerve between the two points of stimulation is estimated ; from 

 these data the rate of transmission of the nervous impulse can be 

 calculated. 



CHAPTER X 



THE POLARISATION OF ELECTRODES AND UNPOLARIS- 

 ABLE ELECTRODES 



Polarisation of Electrodes. Ordinary metal electrodes in contact 

 with a muscle or nerve will be surrounded by lymph, and in this 

 fluid electrolysis will take place during the passage of an electric 

 current. The ions resulting from this electrolysis will be positive 

 and negative respectively ; if, therefore, the circuit of this seat of 



