106 PHYSIOLOGY OF MUSCLE AND NERVE 



In the latter case the muscle twitches with every systole of this organ, 

 thereby proving that a current of action is also generated in cardiac 

 muscle. ^ Similar currents arise in glandular tissue during active secre- 

 tion and in nerves when made to conduct impulses. This phenom- 

 enon also manifests itself in the optic nerve when the retina is stimu- 

 lated by light. 



The Different Phases of the Currents in Muscle. — If an injiu-ed 

 muscle is brought into the circuit of a galvanometer, the needle of 

 this instrument is deflected almost immediately to indicate a negativity 

 in the region of the injury. The indicator remains in this position as 

 long as the injury lasts. The current of injury, therefore, possesses 

 only one period; in other words, it is monophasic in its nature. The 



-Diagram Showing Diphasic Character of Action Current. 

 Phase I and Phase //. 

 A, active portion; R, resting portion; ;S, seat of stimulation; G, galvanometer. The 

 current of action is indicated in each case by the arrows. 



current of action, on the other hand, is diphasic, or rather poly phasic, 

 because the muscle contracts not only in the region stimulated but 

 successively throughout its substance (Fig. 61). Inasmuch as this 

 contraction does not involve its different segments simultaneously, 

 but consecutively in the form of a wave, the electrical variations must 

 display a similar wave-like character. To begin with, the zone 

 nearest the seat of the stimulation is electronegative to the resting 

 zone (Phase I.) A moment thereafter, however, the wave of contrac- 

 tion has reached the opposite end of the muscle (Phase II), whereas 

 the area stimulated first has become inactive. The negativity then 

 becomes centralized in the region far away from the seat of the stimu- 

 lation. In order to follow this progressive wave accurately, the gal- 

 vanometer must first execute a deflection in a direction indicating 

 the negativity of the muscle at the point of stimulation and immedi- 



^ The action current of the heart of mammals has also been demonstrated by 

 A. D. Waller with the help of the capillary electrometer, and by Einthoven by 

 means of the string galvanometer. 



