POLAR EFFECTS OF GALVANIC CURRENT 



55 



currents. These experiments show that the spread of current is assisted by the 

 structure of the nerve ; even with a weak current it takes place for a consider- 

 able distance along an intact nerve, but it is easily blocked if the nerve be tied 

 or crushed. 



Electrotonic effects of constant current on excitability. Take a 

 pair of non-polarisable electrodes and connect with a battery of at 

 least two cells, inserting a rheochord, a commutator, and a mercury 

 key into the circuit (polarising circuit). Another circuit is also pre- 

 pared (exciting circuit), 1 including battery, induction coil, and mercury 

 key in the primary circuit ; the secondary circuit is to have a short- 

 circuit key, with which a pair of ordinary metallic electrodes are 



FIG. 49. To TEST THE POLAR EFFECTS OF A CONSTANT CURRENT ON NERVE EXCITABILITY. 



connected ; these electrodes are brought in contact with the nerve 

 of a muscle-nerve preparation near the muscle. The non-polarisable 

 electrodes, which may be of the boot pattern, are fixed to the 

 myograph cork, but slightly raised above it ; the upper part of 

 the nerve is laid upon them (fig. 49). The record of the muscular 

 contractions obtained is made on a stationary drum. Be careful to 

 keep the nerve moist. 



Place the secondary coil at such a distance from the primary coil 

 that faradisation (Helmholtz modification) just produces a small 

 contraction. Now put in the polarising current (1) in an ascend- 



1 A variation of the experiment is to replace the exciting circuit by a few 

 crystals of salt and wait until the penetration of this begins to excite the nerve 

 fibres. The rheochord also may be dispensed with in the polarising circuit, in 

 which case two Daniell cells will suffice. 



