46 HAND-BOOK OF PHYSIOLOGY. 



situ in the thigh of the opposite leg. The electrodes from an induction 

 coil are placed behind the sciatic nerve of the second preparation, high 

 up. On stimulating the nerve with a single induction shock, the muscles 

 not only of the same leg are found to undergo a twitch, but also those of 

 the first preparation, although this is not near the electrodes, and so the 

 stimulation cannot be due to an escape of the current into the first nerve. 

 This experiment is known under the name of the rheoscopic frog. 



Nerve-stimuli. Nerve-fibres require to be stimulated before they 

 can manifest any of their properties, since they have no power of them- 

 selves of generating force or of originating impulses. The stimuli which 

 are capable of exciting nerves to action, are, as in the case of muscle, very 

 diverse. They are of very similar nature in each case. The mechanical, 

 chemical, thermal, and electric stimuli which may be used in the one case 

 are also, with certain differences in the methods employed, efficacious in 

 the other. The chemical stimuli are chiefly these: withdrawal of water, 

 as by drying, strong solutions of neutral salts of potassium, sodium, etc., 

 free inorganic acids, except phosphoric;, some organic acids; ether, chloro- 

 form, and bile salts. The electrical stimuli employed are the induction 

 and continuous currents concerning which the observations in reference to 

 muscular contraction should be consulted, p. 26, et seq., Vol. II. Weaker 

 electrical stimuli will excite nerve than will excite muscle; the nerve 

 stimulus appears to gain strength as it descends, and a weaker stimulus 

 applied far from the muscle will have the same effect as a somewhat 

 stronger one applied to the nerve near the muscle. 



It will be only necessary here to add some account of the effect of a 

 constant electrical current, such as that obtained from Daniell's battery, 

 upon a nerve. This effect may be studied with the apparatus described 

 before. A pair of electrodes are placed behind the nerve of the nerve- 

 muscle preparation, with a Du Bois Keymond's key arranged for short 

 circuiting the battery current, in such a way that when the key is opened 

 the current is sent into the nerve, and when closed the current is cut off. 

 It will be found that with a current of moderate strength there will be a 

 contraction of the muscle both at the opening and at the closing of the 

 key (called respectively making and breaking contractions), but that 

 during the interval between these two events the muscle remains flaccid, 

 provided the battery current continues of constant intensity. If the cur- 

 rent be a very weak or a very strong one the effect is not quite the same; 

 one or other of the contractions may be absent. Which of these con- 

 tractions is absent depends upon another circumstance, viz., the direction 

 of the current. The direction of the current may be ascending or de- 

 scending; if ascending, the anode or positive pole is nearer the muscle 

 than the kathode or negative pole, and the current to return to the bat- 

 tery has to pass up the nerve, if descending, the position of the electrodes 

 is reversed. It will be necessary before considering this question further 



