428 HANDBOOK OF PHYSIOLOGY. 



by means of the following experiment. The new current produced by 

 stimulating the nerve of one nerve-muscle preparation may be used to 

 stimulate the nerve of a second nerve-muscle preparation. The foreleg 

 of a frog with the nerve going to the 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 preserved 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 prepa- 

 ration, 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 ex- 

 periment 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 very similar 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, effica- 

 cious 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, p. 406. 

 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 

 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 DanielFs battery, 

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

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

 muscle preparation, with a Du Bois Reymond'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 contrac- 

 tions), but that during the interval between these two events the muscle 

 remains flaccid, provided the battery current continues of constant in- 

 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 circum- 



