52 



PKACTICAL 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 

 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 contrac- 

 tion 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. 52.-Diagram of Gal- (Fig. 53). The nerve of preparation B is stimu- 



vani's experiment. Con- , , , , . -, . , , , A , 



traction without metals, lated 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 secon- 

 dary twitch can be obtained if A.\ 

 mechanical or thermal stimuli be 

 used to excite the nerve of pre- 

 paration B. Further, ligature of 

 the nerve B with a moist thread 

 will show that there is no escape 



With a Weak induction-Shock ; the Flo . 5 3.-Diagram of the experiment on 



ligature destroys the physiologi- secondary twitch. 



cal continuity and prevents the passage of the excitatory state but 



not that of an electrical current. 



