404 



GENERAL PHYSIOLOGY 



been put to their utmost practical use in the ingenious methods of 

 electrical stimulation, are the reason why in the special physiology 

 of the vertebrates, wherever the effects of stimulation of definite 

 organs are studied, the electrical stimulus is employed almost ex- 

 clusively. Of the various methods of producing electricity 

 (friction, contact, induction), we employ for stimulating purposes 

 in physiology exclusively the galvanic current, obtained by con- 

 tact or induction, because this offers the greatest advantages by 



reason of its constancy and certainty, its 

 convenient handling and applicability, and 

 the capability of its being finely graduated 

 in intensity and duration. Since the 

 methods of galvanic stimulation have 

 developed to very great complication and 

 delicacy, it will be advantageous briefly 

 to consider some of the most important 

 apparatus. 



As has been seen elsewhere, 1 a galvanic 

 tension arises when two strips of different 

 metals or certain other bodies are dipped 

 into a feebly acid liquid. A strip of copper 

 and a strip of zinc, the lower ends of which 

 dip into a vessel containing dilute sul- 

 phuric acid, while the upper ends project 

 freely into the air, constitute the most 

 primitive form of a galvanic element (Fig. 

 188) ; in it between the two free ends of 

 the zinc and the copper a tension exists of 

 such a kind that the end of the copper is 

 electrically positive, the end of the zinc 

 electrically negative. If the two ends be 

 united by a metallic conductor, such as a 

 wire, at the moment when the union is 

 established, the electrical tension becomes 

 equalised. Since, however, the tension is being constantly renewed 

 at the place of contact of the metals with the liquid, there results a 

 continuous equalisation, which is termed a constant galvanic current. 

 The continuity of copper, wire, zinc, liquid, and copper, forms in a 

 certain sense a closed circuit, in which the current flows. This 

 galvanic current has always the same direction ; outside the 

 liquid it flows from the copper, the positive pole, through the wire 

 to the zinc, the negative pole. In the liquid its direction is 

 evidently reversed ; from the zinc, through the liquid, back to the 

 copper ; but this reversal need occasion no confusion, since it is 

 customary to term the poles outside the liquid the positive and 

 negative poles. The copper is the positive, the zinc the negative 



1 Cf. p. 264. 



FIG. 188. Galvanic element. 

 The free pole of the zinc (-) 

 is joined to the free pole of 

 the copper (+) by a wire ; a 

 circuit is thus formed, in 

 which the direction of the 

 current is indicated by the 

 arrows. 



