26 



ESSENTIALS OF PHYSIOLOGY. 



galvanometer from C to B. When the excitatory process reaches C,' 

 and is still present at B, there is no electrical current and the thread 

 swings back to its original position. This period is extremely short, 

 and almost immediately the excitatory process passes off at B, though it 

 is still present at C ; the point C is now negative to B and the thread 

 swings once more, this time in the opposite direction. Finally the whole 

 muscle ceases to be excited and the thread returns to its resting posi- 

 tion. This diphasic current is known as the current of action. 



If the muscle is injured at the point C, a current flows through the 

 galvanometer from B to C even when the muscle is resting, and is 



called the current of rest 

 or injury. If the muscle 

 is thrown into contraction, 

 the point B becomes elec- 

 trically negative as com- 

 pared with its resting state, 

 and then returns to its 

 * former condition ; the in- 



jured muscle at C does not 

 contract and its electrical 

 condition remains un- 

 changed. The record of 

 the galvanometer therefore 

 shows a single (inonophasic) 



deflection. Since the cur- 

 iiG. 5. Diphasic electrical change in muscle 



(diagrammatic). ' rent of action in this case 



The horizontal line represents the position of the thread flows in the opposite direc- 

 when at rest. 



tion to that of the injury 



current, it is often spoken of as the negative variation of the injury 

 current. 



Thus it is evident that both injured and contracting muscle are 

 negative to normal resting muscle. These changes are abolished by 

 the death of the muscle, and are thus bound up with the chemical 

 changes taking place in a contracting or injured muscle. They are 

 independent, however, of the mechanical shortening, and still occur in 

 an excited muscle even when its power to contract is abolished by 

 steeping it for a short time in distilled water. 



Although it is customary to speak of excited muscle as " negative " 

 to resting muscle, it must be remembered that it is really electro- 

 positive to the resting muscle ; in the same way, zinc is the electro- 

 positive element in a battery. The term "negative" simply means 

 that the current passes through the galvanometer towards the excited 



