70 HUMAN PHYSIOLOGY 



and when this is made to bear upon a smoked surface, 

 either at rest or in motion, a record of the rise and fall is 

 traced. This is an example of what is called the graphic 

 method in physiology. 



Suppose, now, that a muscle has been prepared and 

 suspended above a writing lever. Suitable contacts are 

 provided for the passage of electric currents through the 

 tissue. The experimenter administers a momentary 

 shock. The muscle is seen to twitch, the lever springs 

 upward and falls again. All that can be ascertained 

 by direct observation is that we have to do with a fairly 

 quick and brief response. To learn more than this we 



FIG. 16. Curve of simple muscular contraction. (HowelL) 



must have recourse to additional apparatus. The 

 smoked paper may be borne upon a metal drum turned 

 by clockwork. If the drum is revolved at a brisk rate 

 while the muscle is made to repeat its sharp twitch a 

 curve will be left which can be analyzed with profit. 



The curve traced upon a moving surface when the 

 muscle acts in response to a single stimulus is the curve 

 of a " simple muscle contraction." The speed of the 

 recording surface influences its appearance but not its 

 significance. It will be found to have a definite peak 

 rather than a flat summit. On the average the descend- 

 ing slope of the curve the record of the relaxation 

 will be somewhat longer than the ascending part which 



