114 CHANGE IN FORM IN A MUSCLE WHEN IT CONTRACTS [CH. X. 



cell, coil, keys, and electrodes are applied with the object of stimulat- 

 ing the nerve. 



The key E makes and breaks the primary circuit, but the effect is 

 only felt by the muscle-nerve preparation when the short-circuiting 

 key D in the secondary circuit is opened. 



Instead of the key E it is better to have what is called a " kick- 

 over " key which the cylinder by means of a bar projecting from it 

 knocks over and so breaks the primary circuit during the course of a 

 revolution. The exact position of the writing-point at the moment 

 of break, that is the moment of excitation, can then be marked on 

 the blackened paper. 



Besides the travelling cylinder there are other forms of writing 



FIG. 142. Du Bois Reymond's Spring Myograph. (M'Keudrick.) 



surface. Thus fig. 142 represents the spring myograph of Du Bois 

 Eeymond. Here a blackened glass plate is shot along by the recoil 

 of a spring ; as it travels it kicks over a key, and the result of this, 

 the muscular contraction, is written on the plate. 



The pendulum myograph (fig. 143) is another form. The pen- 

 dulum carries a smoked glass plate upon which the writing-point of 

 the muscle lever is made to mark. The break shock is sent into the 

 muscle-nerve preparation by the pendulum in its swing opening a 

 key in the primary circuit. This is shown in an enlarged scale in BC 

 (fig. 143). 



To keep the preparation fresh during an experiment, it should be 

 covered with a glass shade, the air of which is kept moist by means 



