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A MANUAL OF VETERINARY PHYSIOLOGY 



explain injury currents in muscle is again used here to explain the 

 same currents in nerves. If the nerve be injured, the current of 

 injury flows from the equator to the cut end outside the nerve, 

 and from the cut end to the equator inside the nerve. If a nerve, 

 while exhibiting the presence of a current of injury, be stimulated, 

 the uninjured portion becomes electrically negative to the injured 

 part. There are now two currents in the nerve — viz., the current 

 of injury and the new one just created ; each is flowing in the 

 opposite direction, the result being that the current of injury 

 becomes diminished. This diminution or reduction in the current 

 of injury is termed negative variation. 



In the two observations described the nerve is assumed to be in 

 an injured condition. If now a constant current be passed through 

 an uninjured nerve, a wave of electrical negativity travels along it, 



Fig. 125. — Diagram illustrating the Electric Currents of Injury in 

 Nerve and Muscle (Foster). 



The diagram serves either for nerve or muscle, excepting that the current gh 

 is not present in nerve. The strong currents are shown by the dark lines ; 

 the arrows show the direction through the galvanometer, ab, The equator ; 

 ax, current from equator to cut end ; xa, current within the nerve, cut 

 end to equator. 



and this current is termed the current of action. It will be obvious 

 that it was the presence of the current of action which diminished the 

 current of injury in the previous experiment. Let a nerve be taken 

 and arranged as shown in Fig. 126, A — viz., a constant (polarising) 

 current passed through its middle piece, while a galvanometer is 

 connected with both regions of the nerve outside the anode and 

 kathode poles of the battery. Under these circumstances, a polar- 

 ising current passes through the nerve in the direction of its circuit — 

 viz., from anode to kathode and back to the battery ; but, in addi- 

 tion to this, there is a current in the extra-polar regions, as indicated 

 by the attached galvanometers. These extra-polar currents are 

 termed electrotonic. The piece of nerve beyond the kathode is 

 increased in excitability and conductivity, and this is termed kath- 

 electrotonus. The portion of nerve beyond the anode is decreased 

 in excitability and conductivity, and is in a condition known as 

 anelectrotonus. It will be observed that, though the battery current 



