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TEXT-BOOK OF PHYSIOLOGY. 



at all. Stimulation of the extra-kathodic region, though of increased 



excitability, as shown by the previous experiment, may also fail to 



provoke a contraction, owing to the diminished conductivity of the 



region in the neighborhood of the anode. The impulse on reaching 



this region is blocked in its passage. A similar if not more marked 



decrease in the conductivity may be developed in the region of the 



kathode if the current strength be very great. (See Fig. 55.) 



The Law of Contraction; Polar Stimulation. It was stated in a 



previous paragraph that when a galvanic current of medium strength is 



made to enter a nerve, and when it is withdrawn from the nerve, there is a 



contraction of its related muscle. These are generally known as the make 



and break effects. During the actual passage of the current no effect is 



observed so long as its strength remains uniform. Any sudden variation 



in the strength of the current at once arouses the nerve to activity, as shown 



by a muscle contraction. 



The muscle response to the make and break of the constant current is 

 more or less variable unless the direction of the current as well as its strength 

 be taken into consideration. If the current is made to flow from the central 

 toward the peripheral end of the nerve it is termed a direct, descending, or 

 centrifugal current; if it is made to flow in the reverse direction, it is termed 

 an indirect, ascending, or centripetal current. The strength of the current is 

 determined and regulated by means of a rheocord. 



The make and break of currents of different but known strengths and 

 directions give rise to contractions which occur with more or less regularity. 

 The order in which they occur under these varying conditions of experi- 

 mentation has been determined and tabulated as follows by Pfliiger, and is 

 termed the law of contraction: 



The results as above tabulated are sometimes complicated on the open- 

 ing of the circuit by a series of irregular pulsations of the muscle, an ap- 

 parent tetanus, and long known as the opening tetanus of Ritter, which is 

 attributed to rapid changes in the irritability of the nerve, in the region of 

 the anode. A similar tetanic contraction of the muscle is sometimes ob- 

 served on the closure of the circuit due to continued excitation in the region 

 of the kathode. This is known as the closing tetanus of Wundt or of Pfliiger. 

 All the phenomena of the law of contraction were explained by Pfliiger on 

 the assumption that the current stimulates the nerve only at the one electrode, 

 at the kathode on closing, and at the anode on opening; or, in other words, by 

 the appearance of katelectrotonus or by the disappearance of anelectrotonus, 



