52 PEACTICAL PHYSIOLOGY 



injury-current.' This electrical current produced by injury is, as Gotch 

 pointed out, to be considered as a current of action. These facts can be 

 demonstrated by the following experiments. 



The Rheoscopic Frog. Galvani's Experiment, Contraction without 

 Metals. A long length of the sciatic nerve is dissected in a pithed frog 

 and the muscles of the thigh are exposed and cut across. The trunk of 

 the sciatic nerve is laid along the longitudinal surface of the muscles of 

 the thigh, and then by raising the end of the nerve by a small glass rod 

 the transverse section of the nerve is allowed to fall upon the cut 

 surface of the muscles (Fig. 52). At this moment a twitch of the 



FIG. 52. Diagram of Galvani's experiment. Contraction without metals 



muscles of the leg moves the foot or toes. The circuit of the electric 

 current in the muscle has been completed through the nerve. The 

 section of the muscle-fibres has produced a local contraction of the 

 fibres, and this is accompanied by an electrical change which is 

 sufficient to produce excitation when it is passed through an excitable 

 nerve. 



Secondary Contraction or Secondary Twitch. Two muscle- and 

 nerve-preparations are made ; the nerve of A is so placed upon the 

 muscle B that the cut surface of the nerve lies upon the tendon and 

 its longitudinal surface upon the muscle-fibres (Fig. 53). The nerve 

 of preparation B is stimulated by a weak induction-shock, and thus its 

 muscle is excited and made to contract; the muscle A will also 

 contract. The contraction of the muscle B is accompanied by an 

 electrical current, the * current of action,' which passes through the 

 nerve A and thus produces a contraction in the muscle A. This is 

 not due to an escape of electrical current from the electrodes, for a 

 secondary twitch can be obtained . if mechanical or thermal stimuli be 



