STRUCTURE AND PHYSICAL PROPERTIES OF MUSCLE 537 



current. At the break of the current, an extra current is also pro- 

 duced in the primary coil in the same direction as the battery current ; 

 but the primary circuit being broken, it cannot delay the rapidity of the 

 fall of the battery current. 



When rapidly induced shocks (50 to 100 per second) are required 

 (the so-called faradic or tetanizing current), the primary circuit is 

 rapidly made and broken by means of Wagner's hammer. The wires 

 from the battery are connected to the two bottom screws of the primary 

 coil (Fig. 270). It will be seen that the current passes, via the pillar A 

 and spring H, through the primary coil to the electro-magnet E. 

 This becomes an electro -magnet, and pulls down the piece of steel 

 on the spring H, and thus breaks the circuit. E then, being no longer 

 a magnet, releases the spring hammer, which flies back, and again 

 completes the circuit; and so the process is repeated. Every time the 

 hammer is attracted to the magnet the current is broken, and a break 



FIG. 270. DIAGRAM TO SHOW THE ACTION OF WAGNEK'S HAMMER. 



shock induced; every time it flies back a make shock is induced 

 Here again the make is less in intensity than the break shock. 

 The make-and-break shocks can be equalized by placing a wire 

 from the binding screw (7) to the top binding screw (1) of the primary 

 coil,.and screwing up the top screw S v out of the way, and at the same 

 time screwing up screw S 2 . The current now passes into the primary 

 coil by this wire. E, as before, becomes a magnet, and pulls down 

 the armature. This short-circuits the current back to the battery. 

 There is still left, however, a circuit for the extra break current 

 (7, W, I, PC, E, H, A, 1), and this reduces the strength of the break 

 current in the secondary coil, thereby equalizing the make and break 

 currents (Fig. 271). 



Proof is required to show that a muscle is really stimulated directly, 

 and not indirectly, through the nerve-fibres and nerve-endings in 

 the muscle ? The direct excitability of muscle is shown by the 

 following experiments: (1) Parts of muscles which contain no nerve- 

 fibres for example, the end of the frog's sartorius respond to direct 



