THE INHIBITOR Y NER VES ^ 5 



ductor muscle and a weaker abductor, and found that removal of 

 the one muscle produced a condition of tone in the other. With 

 well-pronounced tone in the adductor he found that weak stimu- 

 lation produces inhibition of he tone of the muscle and the 

 claw opens, strong stimulation produces contraction and closure 

 of the claw. On the other hand with the abductor muscle 

 isolated and in a condition of tone, weak stimulation causes con- 

 traction of the muscle and the claw opens, while strong stimula- 

 tion causes inhibition and the claw closes. The same strength 

 of stimulus, which causes contraction of the one muscle, thus 

 causes inhibition of the other. The stimulus was always applied 

 by electrodes stuck into the limb itself, so that, on the assumption 

 that inhibitory nerves exist apart from motor nerves, each stimulus 

 stimulated four nerves simultaneously. In the case of the 

 adductor muscle the inhibitory fibres are stimulated more strongly 

 than the motor fibres when the stimulus is weak, by increase of 

 strength of stimulus the motors get the upper hand, while in 

 the case of the abductor muscle the motors are supplanted by 

 the inhibitors with a strong stimulation. 



The nerve to the appendage divides into two nerves imme- 

 diately after it leaves the central nervous system, as is shown in 

 Hardy's paper (cp. his figures 1 1 and 1 2). Of these two nerves one 

 is larger than the other, and Marshall found in Homarus vulgaris 

 that, if the larger nerve was cut and its distal end stimulated, 

 the claw closed, whereas if its central end was stimulated, the 

 claw opened reflexly ; with the smaller nerve he obtained the 

 opposite results. Hardy confirms this for Astacus. The larger 

 nerve then is the adductor motor nerve, the smaller the ab- 

 ductor motor nerve. With these two nerves Celesia has re- 

 peated Biedermann's experiment and found that, when the 

 abductor muscle is cut away, stimulation of the abductor nerve 

 causes inhibition of the adductor muscle and vice versa, so that 

 the motor nerve of the one muscle leaves the central nervous 

 system with the inhibitor nerve of the opposing muscle. 



Now the motor nerve consists of fibres which are undoubtedly 

 the axons of some of the large nerve cells so conspicuous in the 

 crustacean nervous system. Where do the inhibitory fibres come 

 from ? Unfortunately at present we do not know sufficiently the 

 structure of the central nervous system in the Crustacea to give 

 a definite answer to this question, but there are certain significant 



