68 PHYSIOLOGICAL PHYSICS. [Chap. vn. 



commutator. When the commutator inclines down 

 towards 3 and 4, direct stimulation is employed ; when 

 it is reversed, indirect stimulation. When the bridge 

 is placed quite horizontal, the arcs touch on neither 

 side ; no current passes, and so the commutator also 

 acts as a key. Now remove the secondary coil of the 

 induction machine along its roadway to some distance 

 from the primary ; incline the commutator bridge so 

 as to stimulate directly, and slowly approximate the 

 secondary coil to the primary, opening and closing the 

 primary key meanwhile. For a considerable time no 

 effect will be produced, and it is not till the secondary 

 is near to the primary coil (probably at a distance of 

 about 16 centimetres on the scale) that contraction 

 of the muscle is noted on opening (interrupting the 

 current), while very likely the secondary will require 

 to be a half nearer the primary coil (8 cc.) before 

 contraction 011 dosing is noted. Now, the key being 

 open, reverse the commutator, so as to send the 

 current to the nerve ; remove the secondary coil, and 

 repeat the manoeuvre of approximating it to the 

 primary, opening and closing the key the while. It 

 will then be found that the secondary coil at a much 

 greater distance from the primary, perhaps 70 cc., 

 gives a shock, on opening, sufficient to cause contrac- 

 tion, and a little nearer produces a closing contraction. 

 By a similar arrangement, the greater stimulating 

 effect of a tetanising current, when applied me- 

 diately by the nerve rather than immediately to the 

 muscle, can be shown. 



Fflueger's trip-2iammer, or fall-hammer. 

 An objection to the accuracy of this comparison be- 

 tween opening and closing shocks is, that one cannot 

 be sure that the opening and closing are effected by 

 the use of an ordinary key with equal suddenness ; 

 for slight differences in the quickness of movement of 

 the key would produce a varying abruptness in the 



