viii CONDUCTIVITY AXD EXCITABILITY OF XERVE 101 



tetanus. If the excitation is weakened again, the opposite effect 

 occurs, i.e. the muscle relaxes. With the gradual disappearance 

 of the tonic contraction, the visible excitation effects appear in 

 one direction only, namely, closure of the claw, i.e. contraction of 

 the muscle. The same thing of course follows in preparations 

 where the adductor muscle from the first manifests no perceptible 

 tonus. In correspondence with the 

 characteristics of this muscle as cross- 

 striated, the changes of form described 

 take place with considerable rapidity. 

 If they are graphically recorded on a 

 slowly-moving surface (Fig. 167), the 1 < i . . 1 . . , , I \ t , . 1 , i 

 curve sinks nearly at right angles at FlG . m.-Tetanising excitation of ad- 

 the beginning of the tetanus, when the ductor muscle of crayfish claw in 



persistent tonic contraction ; suc- 



mUSCle Undergoes Sudden and Often cession of brief inhibitions (relaxa- 



maximal relaxation. At that strength tion) at each excitation of the nerve ' 

 of current with which the inhibitory excitation passes into its 

 contrary, the effects are not seldom in both directions, and many 

 irregularities are manifested. 



In cases where the natural tonus is wanting, the inhibitory 

 effect of stimulating the nerve may sometimes be demonstrated if 

 the relaxed and resting muscle is artificially thrown into persistent 

 or rhythmically interrupted excitation. This is easily effected by 

 including a metronome in the secondary circuit, as well as the 

 vibrating hammer of the coil, so that groups of induction shocks at 

 any required rhythm may be sent into the adductor muscle by 

 means of two platinum points thrust through the chitinous sheath 

 of the claw. Regular rhythmical contractions are thus produced, 

 which can be affected by simultaneous excitation of the nerve in 

 the same way as the natural heart-beat, by excitation of the vagus. 

 If, in the preparation in question, the adductor muscle is thrown 

 into moderate persistent contraction by direct rhythmical excita- 

 tion, and only oscillates, as it were, about its new position of 

 equilibrium, in the rhythm of the metronome, a more or less rapid 

 relaxation occurs at the commencement of inhibitory excitation of 

 the nerve by tetanising induction currents (as in natural tonus). 

 This is graphically expressed as a shallow inflection, or as a rapid 

 fall of the curve, corresponding with complete relaxation of the 

 muscle. In the first case (as nearly always happens with minimal 

 excitation of the nerve), the magnitude of the rhythmical variations 



