chap, viii.] THE LAW OF CONTRACTION. 87 



Reverse c, so that the electrotonising current is 

 now up the nerve. Proceed again to stimulate the 

 nerve, and this time approximate the secondary coil 

 just till the muscle becomes tetanised. Then close 

 the constant circuit. The stimulus is now in the 

 region of anelectrotonus (i.e. of diminished excita- 

 bility), and consequently the stimulus, before just 

 sufficient to tetaiiise, is now no longer sufficient ; the 

 telegraph signal drops ; tetanus disappears. 



Proceed in the same way with the other two cases. 

 Reverse the double commutator, to change the position 

 of stimulating electrodes, which must now be the wires 

 1 2, distant from the muscle, and arrange commu- 

 tator c to get a downward constant current. Stimu- 

 late till tetanus affects the muscle, then electrotonise; 

 anelectrotonus (diminished excitability) is established 

 in the region where the stimulus is applied, and so 

 the stimulus is no longer sufficient ; tetanus dis- 

 appears. Send, lastly, an upward current; you 

 stimulate now in the region of increased excitability, 

 and consequently tetanus appears. Electrotonus also 

 alters the electromotive force of a nerve. (Refer to 

 chapter xi.) In the way thus detailed the student 

 can satisfy himself that the excitability is increased in 

 the neighbourhood of the negative pole when a nerve is 

 made electrotonic. 



L,aw of contraction. Another use of the 

 rheocord is for aiding in the study of the effects of 

 the interruption of the constant current upon a nerve. 

 As already noted (page 65), the passage of a continuous 

 stream through a nerve has 110 apparent effect. On 

 opening (breaking) the circuit, however, or on closing 

 it, varying effects result, sometimes a contraction 

 occurring on opening and none on closing, and vice 

 versa, and other differences. These variations have 

 been studied by various observers, Pfaff, Ritter, 

 Nobili, Heidenhain, Pflueger, and others. As a result 



