24 



PRACTICAL PHYSIOLOGY 



spring, and the platinum wire of the tuning-fork again touches the 

 mercury, thereby making the circuit again. 



To record the contraction of a muscle in response to a single maximal 

 induction- shock, the apparatus is set up in the following way (Fig. 31). 



Fio. 30. Diagram of the chronograph circuit. 



Connect one pole of a Daniell cell to one top binding-screw of the 

 primary coil, and the other binding-screw of the coil to a binding-screw 

 on the base of the stand of the drum. The current passes through the 

 metal work of the stand to a metal striker carried beneath the drum on 

 its axle. As the drum revolves this striker touches a strip of naked 

 wire attached to, but insulated from, the rest of the stand. The 



binding-screw in connec- 

 tion with this naked wire 

 is connected to the other 

 pole of the battery. It is 

 only when the striker and 

 naked wire are in contact 

 that the primary circuit is 

 completed. 



A sciatic and gastroc- 

 nemius preparation is 

 made and attached to the 

 myograph-lever, which is 

 weighted near its axis with 10 or 20 grams, and should then be 

 horizontal. The nerve is laid across the electrodes coming from 

 the Du Bois key, and the secondary coil is arranged to give maximal 

 induction-shocks. A tuning-fork giving 100 complete vibrations per 

 second is arranged to write just beneath the myograph lever. 

 Before the two writing points are brought into contact with the 

 smoked surface, the drum should be made to revolve in order to 

 see that it will rotate away from the writing points and at a suffi- 



Fio. 31. Diagram of the apparatus for recording a 

 single muscular contraction. 



