528 HANDBOOK OF PHYSIOLOGY. 



gastrocnemius cut long is placed upon a glass plate, and arranged in such way 

 that its nerve touches in two places the sciatic nerve, exposed but preserver 

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

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

 On stimulating it 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 themselves 

 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 very similar in each case. The mechanical, chem- 

 ical, 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, 

 chloroform, 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. Weaker elec- 

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

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

 far from the muscle will have the same effect as a 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 current, such as that obtained from a DanielPs battery, upon a 

 nerve. This effect may be studied with the apparatus described before. 

 A pair of electrodes is placed behind the nerve of the nerve-muscle prep- 

 aration, with a Du Bois Raymond's key arranged for short circuiting 

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

 rent 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 

 current 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 

 contractions is absent depends upon another circumstance, viz., the 

 direction of the current. The direction of the current may be ascending 

 or descending: if ascending, the anode or positive pole is nearer the 

 muscle- than the cathode or negative pole, and the current to return to 



