160 ELECTRO-PHYSIOLOGY CHAP. 



V at closure only and not at opening of the circuit, whatever the 

 ' direction of the current," provided only that it is not too strong. 

 Valentin, who characterises this reaction as " the true law of con- 

 traction in vigorous, unaltered, living nerve," certainly carried out 

 his experiments under conditions that did not admit of a fail- 

 estimate of current distribution, since he introduced (metal) elec- 

 trodes into the thigh of the intact animal. This, however, 

 matters the less, since Claude Bernard and Schiff had already 

 arrived at the same results on stimulating the exposed nerves 

 (connected with the central organs) of vertebrates, of different 

 classes. Bernard already inclined to the view that the nervous 

 centres exert a special influence upon the efferent nerve-trunks, 

 thus keeping up the normal excitability, and enabling them to 

 fall into excitation at the entrance only of (not unduly strong) 

 currents. The following remark seems at all events to bear such 

 an interpretation : " Le nerf moteur tire ainsi ses proprietes de la 

 nioelle. II les perd a 1'air ; mais il peut les reprendre, pourvu 

 qu'il communique encore avec le centre nerveux." At the same 

 time, the experiment adduced as evidence hardly justifies the 

 conclusion. It is merely that a partially isolated frog's sciatic 

 regains its normal excitability, when the part is moistened, after 

 previous alteration from drying. 



The idea that the failure of the opening twitch, on exciting 

 undivided nerves with even strong currents, is due to an inhibitory 

 impulse from the central organ, finds definite exposition in a recent 

 work by T. Eumpf (33). The experiments were mostly carried out 

 on the same preparation as was employed by Bernard. The 

 sciatic nerve forms the only connection between one of the legs 

 and the otherwise intact body of the frog. From the fact that 

 here, " in the nerve connected with the central organ, the 

 opening twitch of the ascending current occurs much later 

 (i.e. with stronger currents) than in that separated from the 

 central organ " as appears still more plainly when the spinal 

 cord is cooled by external application of some freezing mixture, 

 Rurnpf concludes that " constant effects are visible in the motor 

 nerve connected with the central organ (as expressed in altera- 

 tions of electrical excitability) which cannot be demonstrated in 

 a nerve separated from its centre, since in this case the opening- 

 twitch either appears simultaneously with, or shortly after, the 

 closure twitch." The latter is "not modified" by the section. 



