50 PRACTICAL PHYSIOLOGY 



(2) Now move the lever to another part of the drum, shorten 

 the spring, and take another trace of the same duration. 



(3) Repeat several times, shortening the spring and each time 

 noting its length. 



(4) Finally connect up the Neef 's hammer of the induction coil 

 a very short rapidly- vibrating spring (p. 83) and take another tracing. 

 Take a time trace of intervals of r \ T sec. by means of an electro- 

 magnetic marker. Fix the tracings and study the results of a 

 succession of stimuli, and formulate your conclusions from these 

 results. 



(Read Physiological Tetanus in Text Book.) 



H. CONTRACTION OF HUMAN MUSCLE. 



If frogs are not available, human muscle may be used by the 

 following method. 



The wires from the short-circuiting key in the secondary circuit 

 (Fig. 28, 5) are fitted with electrodes similar to those used for stimu- 

 lating nerve and muscle through the skin (p. 35,) and covered 

 with chamois leather. Before using the electrodes they are soaked 

 in a saturated solution of common salt. 



The angled lever of the frog-board has attached to the highest 

 hole in the vertical limit a thread rather longer than the frog-board 

 with a slip noose at the end. 



The recording limb of the lever should be short and a weight of 

 40 or 50 grms. should be attached to it. 



A wooden platform, readily made from an old box, is placed on 

 the table behind the frog-board and the top should be at the height 

 above the ground of the subject's elbow when standing beside the 

 table. 



The forearm is bared and its dorsal aspect rested upon the plat- 

 form with the fingers in the natural position of semi-flexion. A 

 piece of tape fixes the first phalanges to the board and another 

 piece is placed round the forearm to steady it. The noose is now 

 placed round the terminal phalanx of the middle or ring finger, and 

 by altering the position of the arm or the platform the thread is 

 tightened till the recording limb of the lever is horizontal. 



The skin below the elbow is well moistened with salt solution 

 and the subject holds the flat electrode upon it. 



The experimenter finds the motor point for the flexor sublimis 

 digitorum acting upon the finger used and marks it with a skin-pencil 

 so that the electrode may, each time it is used, be applied at exactly 

 the same spot and with the same pressure. 



Having adjusted the secondary coil so that a good contraction to 

 breaking the primary circuit is obtained, the point of the lever is 

 swung against the drum. The drum is started and when revolving 

 uniformly the short circuiting key in the secondary is opened and a 

 contraction recorded. The moment of stimulation is marked in 



