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TEXT-BOOK OF PHYSIOLOGY. 



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

 if the current strength be very great. (See Fig. 54.) 



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 strength is made to enter a nerve, and when it is withdrawn from the 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



opening of the circuit by a series of irregular pulsations of the muscle, 



an apparent 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 observed on the closure of the circuit due to 



continued excitation in the region of the cathode. This is known 



as the closing tetanus of Wundt. All the phenomena of the law of 



vV contraction were explained by Pfliiger on the assumption that the 



vjf current stimulates the nerve only at the one electrode^aj^ the cathode 



\ on closing, and at the anode on oj>ening\pr, in other words, by the 



appearance of catelectrotonus or by the aisappearance^f ~anelect ro- 



tonus, both conditions being attended by a rise of excitability not, 



however, by the opposite changes. It is further assumed that the 



