I2O 



INSTRUMENTS. 



[CH. X 



The writing surface is again a travelling cylinder tightly 

 covered with smoked glazed paper. The rest of the apparatus 

 shows how cell, coil, keys, and electrodes are applied with the 

 object of stimulating 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 



Fig. 142. Du Bois Reymond's spring myograph. (M'Kendrick.) 



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 

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

 Bois Reymond. 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. Here the 

 movement of the pendulum along a certain arc is substituted for 

 the clockwork of the cylinder, or the spring of Du Bois Reymond. 

 The pendulum carries a smoked glass plate upon Avhich the 

 writing-point of the muscle lever is made to mark. The break 



