166 EXPERIMENTAL PHYSIOLOGY. 



(b.) Gradually approximate the secondary coil, open the 

 short-circuiting key, and break the primary circuit by means 

 of the Morse key in it. Observe when the first feeble con- 

 traction is obtained = minimal contraction. Make the primary 

 circuit, there is no contraction. The break shock is, there- 

 fore, stronger than the make. Record under each contraction 

 whether it is a make (M) or break (B) shock, and the dis- 

 tance in centimetres of the secondary from the primary coil. 

 Move the drum a short distance with the hand, the lever 

 will inscribe a horizontal or base line, the abscissa. 



(c.) Gradually approximate the secondary spiral, and from 

 time to time test the effect of the make and break shocks, 

 after each test moving the cylinder with the hand, and 

 recording the result as to M or B, and the distance in centi- 

 meters of the secondary from the primary coil. After a time 

 a make contraction appears, and on pushing up the secondary 

 coil the make contraction becomes as high as the break. 



(d.) Increase the stimulus by bringing the secondary 

 nearer the primary coil, and notice that the contractions do 

 not become higher = maximal contraction. In each case keep 

 the make and break contractions, obtained with each strength 

 of current, close together. Their relative heights can then 

 be readily compared. 



B. Arrange the experiment as in A, but allow the cylinder to 

 revolve at a moderate speed, 25-30 centimetres per second ; the 

 writing-style records an even horizontal line = the abscissa. 



(a.) Select a strength of stimulus which is known to cause 

 a contraction, and while the cylinder is revolving, cause the 

 muscle to contract either by a make or break shock. Study 

 the characters of the " muscle -curve." 



(6.) Vary the velocity of movement of the cylinder, and 

 observe how the form of the curve varies with the variation 

 in velocity of the cylinder. 



(c.) Remove the tracings in A and B, and varnish them. 

 In this case the moment of stimulation is not recorded. 



