58 



THE CONTRACTILE TISSUES. 



constant current, as we shall call it, and the induced current obtained from 

 the constant current by means of an induction coil, as it is called ; for 



FIG. 11. 



A Muscle-nerve Preparation : TO, the muscle, gastrocnemius of frog; n, the sciatic nerve, all 

 the branches being cut away except that supplying the muscle ; /, femur; cl., clamp ; t. a., tendo 

 Achillis ; sp. c., end of spinal canal. 



the physiological effects of the two kinds of current are in many ways 

 different. 



It may, perhaps, be worth while to remind the reader of the following facts : 

 In a galvanic battery, the substance (plate of zinc, for instance) which is acted 

 upon and used by the liquid is called the positive element, and the substance 

 which is not so acted upon and used up (plate, etc., of copper, platinum, or carbon, 

 etc. ) is called the negative element. A galvanic action is set up when the positive 

 (zinc) and the negative (copper) elements are connected outside the battery by 

 some conducting material, such as a wire, and the current is said to flow in a cir- 

 cuit or circle from the zinc or positive element to the copper or negative element 

 inside the battery, and then from the copper or negative element back to the zinc 

 or positive element through the wire outside the battery. If the conducting wire 

 be cut through, the current ceases to flow ; but if the cut ends be brought into 

 contact, the current is re-established and continues to flow so long as the contact 

 is good. The ends of the wires are called " poles," or when used for physiological 

 purposes, in which case they may be fashioned in various ways, are spoken of as 

 electrodes. When the poles are brought into contact or are connected by some 

 conducting material, galvanic action is set up, and the current flows through the 

 battery and wires; this is spoken of as "making the current" or "completing or 

 closing the circuit." When the. poles are drawn apart from each other, or when 

 some non-conducting material is interposed between them, the galvanic action is 

 arrested; this is spoken of as "breaking the current" or "opening the circuit." 

 The current passes from the wire connected with the negative (copper) element in 

 the battery to the wire connected with the positive (zinc) element in the battery ; 

 hence, the pole connected with the copper (negative) element is called the positive 

 pole, and that connected with the zinc (positive) element is called the negative pole. 

 When used for physiological purposes the positive pole becomes the positive elec- 



