4 8 



PRACTICAL PHYSIOLOGY 



secondary circuit is to have a short-circuit key, with 

 which a pair of ordinary metallic electrodes are 

 connected ; these electrodes are brought in contact 

 with the nerve of a muscle-nerve preparation near 

 the muscle. The non-polarisable electrodes are 

 fixed to the myograph cork, and the upper part of 

 the nerve is laid upon them. The record of the 

 muscular contractions obtained is made on a sta- 

 tionary drum. Be careful to keep the nerve moist. 



FIG. 31. To TEST THE POLAR EFFECTS OF A CONSTANT CURRENT ON NERVE 

 EXCITABILITY. 



Place the secondary coil at such a distance from 

 the primary coil that the break induction shock 

 just produces a contraction. Now put in the polar- 

 ising current first in an ascending and secondly in 

 a descending direction, and determine the effect of 

 its poles in diminishing or increasing the excitabil- 

 ity of the nerve as tested by the submaximal stim- 

 ulus employed. 



Closing and opening tetanus; Pfliiger's law 

 of contraction. Using the same apparatus (but 



