186 ELECTRO-PHYSIOLOGY CHAP. 



Similar experiments on the atonic abductor muscle show 

 that, apart from other differences to be discussed below, it is, 

 as a rule, excited by much weaker currents than the adductor 

 muscle, while strong currents under some conditions prove wholly 

 ineffective, at other times discharging contractions considerably 

 weaker than those from currents of lower intensity. This last 

 paradox, however, is not invariable, and cannot even be termed 

 a frequent occurrence. 



Direction of current seems to be of importance in all experi- 

 ments on the muscles of the claw, inasmuch as the closing excita- 

 tion appears in the majority of cases with ascending, rather than 

 with descending currents, while the contrary is true of the 

 opening stimulation. The cause of this reaction must be sought 

 less in any special property of the nerve-fibres than in the fact 

 that with the above conditions of experiment, current density at 

 the two electrodes is not equal, but is less at the contact that 

 lies towards the periphery than at the central contact. This is 

 due to the form of the joint to which the (thread) electrodes 

 are applied, since the diameter of the joints increases considerably 

 towards the claw. The above difference may be neutralised, or 

 even reversed, by merely passing the threads as near as possible 

 to the nerve, which runs along the inner surface of the limb, or 

 by using a more basal joint for excitation. 



All doubt as to the validity of Pfliiger's law of excitation for 

 the nerves of the adductor as well as the abductor muscle is 

 removed by simply excluding the central electrode, as will be 

 shown below. 



While in atonic muscles treated by the above method the 

 effects of excitation by the constant current are tolerably uniform, 

 there is, along with conformity of detail, a surprising variety of 

 effect, on exciting preparations from either of the two claw 

 muscles, when there is a more or less well -developed tonus. 

 This is intelligible since each single stimulation affects the 

 muscle in an exactly contrary direction, and thus, as will be 

 shown, produces opposite changes of form. Excitatory or inhibi- 

 tory effects preponderate according to the state of the prepara- 

 tion, and the strength and direction of the exciting currents. 



The preparations of the adductor muscle which most con- 

 veniently show the dependence of inhibitory and excitatory 

 action from the constant current, upon its direction and in- 



