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A TEXTBOOK OF PHYSIOLOGY 



and B at rest, there is a current of action through the galvanometer 

 from B to A; when both A and B are active, there is no deflection. 

 When B is active and A at rest, there is a current through the gal- 

 vanometer from A to B. 



If the transmission of the active state from A to B is quick, the 

 isoelectric interval is naturally short; if it be long, the interval is 

 correspondingly increased. Thus, the rate of transmission of the 

 wave of activity may be measured. In the nerve trunk of a frog it 

 is about 30 metres per second; in striated muscle about 1 metre per 

 second; in the frog's ventricle about ^ metre per second. 



J?IG. 303. TYPICAL EXCURSION OF SARTORIUS MUSCLE TO SINGLE INDUCTION TWITCH. 



(Keith Lucas.) 



Read right to left. 



In the heart, the current of action induces a triphasic variation. 

 This can probably be explained as follows : Suppose one non-polarizable 

 electrode (B) is placed on the base of the ventricle, another (A) on the 

 apex. The excitatory wave enters the base of the ventricle by the 

 A.-V. bundle. B is now negative to A. It passes then to the apex. 

 A is now negative to B. The ventricles contract in such a mamiei 

 that the apex finishes contracting before the base. The blood is 

 wrung out from the ventricle, and the muscle round the arterial orifices 

 is the last to contract ; B finally, therefore, becomes negative to A. 

 The response of the heart in terms of negativity is therefore B, A, B 

 (base, apex, base). 



Cutaneous Currents. Normally, the skin of all vertebrate animals 

 is traversed by an electric current from without inwards (Fig. 305). This 

 current appears to be caused mainly by the action of the cutaneous 

 glands, or of active secreting single cells in the skin. When the pad 



