ADVANCED EXPERIMENTAL PHYSIOLOGY 



311 



portion. The entire length of the sciatic nerve is dissected out, and 

 the gastrocnemius muscle is connected with the lever of a myograph ; 

 the drum is arranged for rapid revolution, and a maximal shock is to be 

 used for excitation. The latency of the muscular contraction (Chapter 

 III., p. 22) is then determined, first for stimulation by the upper pair of 

 electrodes, the lower pair being short-circuited by closure of its Du Bois 



FIG. 189. Diagram of the experiment on the rate of transmission of a nervous 



impulse. 



key ; then the experiment is made with the lower pair of electrodes for 

 the exciting point. The time of this latency is determined by recording 

 underneath the curves the vibrations of a tuning fork with 100 vibra- 

 tions per second ; the difference in time between the moment of 

 stimulation and the resulting contraction in the two cases represents 

 the time taken for the nervous impulse to pass along the length of nerve 

 between the two pairs of electrodes (Fig. 189). This piece of nerve is 

 measured in millimetres, 1 and then the velocity of the transmission of 

 the nervous impulse is calculated. 



For the accurate determination of the rate of propagation of a nervous 

 impulse a very rapid rate of movement of the recording surface is 

 required; for this reason the spring-myograph (Fig. 179, p. 299) or the 

 pendulum-myograph may be used with advantage in the place of the 

 drum. 



(b) In the Motor Nerves of Man. The velocity of the transmission 

 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 

 brachiai nerves. These electrodes are connected with the secondary 



1 There is a millimetre scale upon the slide of the induction-coil. 



