102 



ELECTRO-PHYSIOLOGY 



CHAP. 



usually remains unaltered, or at most undergoes some insignificant 

 changes. With stronger excitation the result is otherwise. There 

 will then be, along with the relaxation of the muscle and conse- 

 quent fall of the curve, a perceptible and often considerable 

 diminution in the height of the individual contractions, which 

 does not necessarily entail an alteration in rhythm. This may be 

 carried so far as to render the changes of form in the muscle quite 

 imperceptible at the time of greatest relaxation, or at most in- 

 dicated as slight undulations in the curve (Fig. 168). The tracings 

 obtained from such inhibitory effects not infrequently exhibit a 

 superficial resemblance to kymographic curves, which show the 

 inhibitory action of the excited vagus upon cardiac movements. 



FIG. 168. Inhibition of artificial contractions of adductor muscle 

 of crayfish claw owing to stimulation of the nerve. The 

 contractions were evoked by direct, rhythmic tetanisatious 

 of the muscle. 



FIG. 169. As in Fig. 168. 

 Predominant diminution 

 of single contractions. 



From these observations we learn that diminution of the 

 artificial rhythmical contractions proceeds pari passu with the 

 relaxation of the muscle, and the same fact is even more evident 

 in cases where the muscle has time to relax completely between 

 two consecutive stimuli. The active inhibitory action then 

 betrays itself only by a more or less considerable diminution 

 of the single twitches, or (more correctly) short tetani (Fig. 169). 

 Such a series of curves reminds us directly of the demonstrations 

 of Heidenhain and Lowit, on the effect of minimal vagus-excitation 

 on the rhythmical contractions of the frog's heart, where the 

 first effect of inhibition is a diminution of the single beats. 



The reaction thus described in the muscles of the crayfish 

 claw (which can hardly be explained otherwise than by the 

 antagonistic working of two opposite kinds of fibres passing by 

 the same nerve-trunk into the adductor muscle) is by no means 



