78 PEACTICAL PHYSIOLOGY 



coil of an inductorium, and in the primary circuit is interposed the 

 1 trigger ' key of the spring-myograph. 



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. 25), and then the experi- 

 ment 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 XVIII. (Advanced). 



THE POLARISATION OF ELECTRODES AND UNPOLARISABLE 

 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 chemical and 

 electrical change be suddenly made or broken, a shock will be produced, 

 for the wires of the electrodes surrounded by the electrolysed fluid will 

 form a minute battery. This can be demonstrated by the following 

 experiment : A pair of electrodes, connected with a Du Bois key, is 

 placed under the sciatic nerve, which has been exposed in the thigh of 

 a pithed frog. Making or breaking the circuit causes no contraction. 

 The two wires of a Daniell battery are connected with each side of the 

 Du Bois key, and the current is allowed to pass through the nerve for 

 several seconds. Then these two wires are rapidly disconnected from 

 the battery and key ; the key is closed and opened, and each time 

 a contraction of the muscles of the leg is caused. This make and break 

 can be repeated several times with a similar result, until the polarisation 

 has disappeared. 



This experiment shows the necessity of unpolarisable electrodes in 

 experiments upon the effects produced in nerve and muscle by the 



