30 PRACTICAL PHYSIOLOGY 



tance of the secondary from the primary the break 

 shock produces its full effect i.e., causes a full con- 

 traction when the drum is made to revolve. Do 

 not allow the muscle to be fatigued by many excita- 

 tions before recording its contraction. 



Now bring the lever point so as lightly to touch 

 the blackened paper, using the stop of the myo- 

 graph stand to prevent the possibility of the point 

 pressing too hard against the paper. When the 

 stop is used in this way the lever point can be 

 removed at any time from the paper, and brought 

 back again so as to press with exactly the same 

 force as before ; it is therefore absolutely essential 

 to make use of the stop in all recording experi- 

 ments in which comparisons of different curves 

 upon the same surface have to be made. 



Start the drum revolving, but keep the short- 

 circuit key closed so that no stimulus reaches the 

 nerve ; the lever point will describe a horizontal 

 line (abscissa). Whilst the drum is still revolving 

 open the short-circuit key, but close it again the 

 instant the muscle has contracted ; immediately 

 afterwards remove the lever point from the drum 

 before this has had time to perform another revolu- 

 tion. A simple muscle curve will thus be described. 



To mark the point of stimulation, move the 

 drum slowly round by hand until the projecting 

 pin just touches the spring where contact is 

 made ; bring the lever point against the smoked 

 surface as far as the stop will allow, and raise 

 the lever about half an inch by the finger. The 

 distance between this mark, which indicates the 



