XXXVI.] ISOTONIC AND ISOMETRIC CONTRACTIONS. 203 



electrodes, arrange the weighted writing-lever to write on the 

 drum. 



(e.) Arrange the lever of a chronograph (vibrating 100 times per 

 second and actuated by a Grove's cell in circuit with a tuning-fork, 

 T.C time-circuit) so that the one writing point records exactly 

 under the other. 



Make base lines and ordinates muscle-lever and time-lever 

 on the cylinder to mark the relative positions of the two writing 

 points, or cause one to write exactly over the other. 



(d.) Adjust the position of the break key in order to have the 

 tracing near the middle of the paper and not near where it is 

 gummed. Open the short-circuiting key, set the chronograph 

 vibrating, and the cylinder in motion during one revolution. When 

 the striker (8) comes in contact with support (I.S) a break 

 induction shock is obtained, and the muscle records a simple muscle- 

 curve. Close the short-circuiting key. 



(e.) Eecord the moment of stimulation by bringing S into contact 

 with the style on I.S. The distance between this point and the 

 beginning of the curve indicates the latent period. 



(/.) Study the "muscle-curve" (fig. 121), noting particularly 

 the latent period, the ascent and descent. The latent period may 

 be represented by a distance of 4 or 5 millimetres, but this delay 

 does not represent the actual latent period, which is really much 

 shorter. The long latent period is really largely due to the 

 apparatus and therefore instrumental. Estimate, by means of the 

 tuning-fork vibrations, the duration of each of the phases. 



LESSON XXXVI. 



ISOTONIC AND ISOMETRIC CONTRACTIONS- 

 WORK DONE HEAT-RIGOR. 



1. Isometric v. Isotonic Contraction (Fick). In the ordinary 

 way of recording a simple muscular response or twitch, as just 

 described, a light lever (with a light weight attached) records its 

 movements, so that the muscle is constantly stretched by and con- 

 tracts against a small constant resistance during its contraction. 

 Such a curve is isotonic (fig. 121). 



If, however, the muscle contracts by pulling on a strong spring of 

 great resistance, such a spring, for example, as requires about 500 

 grams to bend it slightly, then the curve obtained is isometric. 

 The curves obtained by clinical dynamometers are of this class. 



