THE MUSCULAR SYSTEM 395 



anode, pole. Within the battery the current flows from zinc to 

 copper, outside the battery — viz., between the poles, it flows from 

 copper to zinc. The circuit is completed by bringing the poles 

 together, known as ' making '; it is broken by separating the poles, 

 known as ' breaking.' When this current is passed through a 

 galvanometer — an instrument which detects the existence, direction, 

 and intensity of currents — it flows through this apparatus from the 

 positive to the negative pole. So much confusion results from the 

 use of the terms ' positive ' and ' negative ' — which, as we have 

 seen, have two opposite meanings, depending upon whether the 

 current is passing through the battery or through the poles — that 

 Waller has proposed that physiologists should call the copper 

 ' galvanometrically positive,' and the zinc ' galvanometrically 

 negative.' in the same way that physicists employ the terms ' electro- 

 positive ' and ' electro-negative ' to indicate the positive and 

 negative elements respectively. 



The induced or interrupted current is produced by passing a 

 galvanic current through an induction-coil (D, Fig. 113). This 

 consists of a primary coil, pr c, connected with the Daniell's cell, E, 

 and a secondary coil, sc D, capable of a sliding adjustment near to 

 or away from the primary coil. The nearer the coils are together, 

 the stronger the current. To the secondary coil wires are attached 

 conveying the induced current. Between the battery and the 

 primary coil is introduced an appliance for making or breaking the 

 current. This may be done by hand, when single shocks are re- 

 quired, by means of the key F, Fig. 113, or automatically by means 

 of an interrupter, when a rapid succession of make and break 

 shocks are required. This may be seen in Fig. 113, to the right of 

 the standard supporting the primary coil. 



If a suitable muscle preparation be connected with a galvanic 

 battery and a weak constant current passed through it, at the 

 moment of ' making ■ the muscle gives a twitch, and nothing more 

 occurs to the eye, though the current continues to pass. If the 

 current be made stronger a twitch occurs both at ' make ' and 

 ' break,' and if the current be sufficiently powerful it will be 

 observed that the contraction at ' make ' is more vigorous than 

 that occurring at ' break. ' This reaction takes place more obviously 

 with striated than with plain muscle. The contraction which 

 occurs at ' make ' starts from the negative pole, and is known as 

 the closing contraction ; that which occurs when the current is 

 broken starts at the positive pole, and is known as the opening 

 contraction. This is described as ' the laws of polar stimulation/ 



When a muscle-nerve preparation is stimulated by a single 

 induction shock the contraction which occurs at ' break ' is 

 stronger than that taking place at ' make.' This appears to be an 

 opposite condition to the effect produced by a constant current ; 

 as a matter of fact, it is not, but is due to an ' extra current ' 

 developed in the primary coil, which, being in the opposite 

 direction to the battery current, renders the ' make ' stimulus 

 ineffective. 



The efficiency of an electric current depends upon the rate of 



