100 ELECTRO-PHYSIOLOGY CHAP. 



contract and relax again, after which follows the neutral period, 

 or else the relaxation of the abductor is immediately followed by 

 the contraction of the adductor (Fig. 166). 



Although the alternation between the antagonist muscles is thus 

 by no means such that the excitation of the one must inevitably 

 exclude that of the other, this frequently is the case. And with- 

 out exception, during maximal excitation of the adductor muscle, 

 the abductor is quiescent, and vice versa. 



The analogous effect of mechanical and chemical stimulation 

 may be demonstrated here, as in the ordinary Bitter- Eollett 

 phenomenon. It is always plain that the action of the abductor 

 muscle preponderates immediately after cutting off the claw, 

 while at the moment of amputation there is a rapid and transi- 

 tory adduction. 



Obviously there is here a complicated action of the mechanical 

 stimulus upon the nerves of both muscles, which could only be 

 analysed after further investigation ; no less striking is the fact 

 that, in the majority of cases, chemical excitation of the claw- 

 nerve (by dipping a fresh section of the limb into concentrated 

 NaCl solution) causes the action of the abductor to preponderate, 

 although the adductor may be thrown into violent contraction by 

 the same stimulant, as appears more especially when the abductor 

 has previously been divided. 



The innervation of the antagonist muscles of the crayfish 

 claw is further complicated by the fact that each of the two 

 muscles is undoubtedly supplied by inhibitory as well as by 

 motor nerve-fibres, and these as regards relations of excitability- 

 behave in a diametrically opposite manner to the motor nerves. 

 Both adductors and abductors usually exhibit a kind of tonus, 

 which appears most plainly after dividing the antagonist muscle. 

 If in such a case (after cutting through the abductor) the nerve 

 of the limb is excited with alternating tetanising currents, while 

 the secondary coil is gradually pushed up to the primary, the first 

 effect of exciting the nerve will invariably be abduction of the 

 claw, which under the circumstances can only be caused by the 

 relaxation and consequent elongation of the adductor muscle. If 

 the excitation is strengthened by gradual approximation of the 

 coils, the same effect increases at first, until, at a certain and 

 generally considerable intensity of current, each excitation causes 

 a vigorous adduction of the claw which lasts throughout the 



