XLI.] FATIGUE OF MUSCLE. 223 



One of the most important points in connection with the use of tliis instrument 

 is to keep the surface of the mercury clean and bright. This is necessary in 



Fig. 146.— Magnetic Interrupter with Tuning-Fork, as made by the Cambridge Sclentiflo 

 Instrument Company. 



order to have the successive shocks of equal intensity. Kronecker has devised 

 such an apparatus. The vibrating rod is so adjusted that stimuli from I to 

 50 or 60 per second can be obtained therewith. 



LESSON XLI. 

 FATIGUE OF MUSCLE. 



1. Fatigue of Excised Muscle. 



(a.) Arrange an induction coil for break shocks, hut interrupt 

 the primary circuit automatically by means of the drum key (fig. 

 120). 



(h.) Fix a nerve-muscle preparation on a crank-myograph, with 

 a long lever and a weight of 40-50 grams, lay the nerve over the 

 electrodes from the short-circuited secondary coil, and let the lever 

 record on the drum. A break shock is obtained each time the 

 drum revolves. The myograph should be supported on a tangent 

 stand. If a tangent support be used for the muscle-lever, then, 

 although the muscle contracts at each revolution of the cylinder, 

 one may record every tenth or fifteenth contraction just as one 

 pleases (fig. 147). 



(c.) Observe that the height of the curves falls, while their 

 duration is longer. In nearly every case fatigue- curves from muscle 

 show a "staircase" character (fig. 148), the second curve being 

 higher than the first one, and the third than the second. 



2. Fatigue-Curve of Excised Muscle. — (a.) Use a slow-revolving drum on 

 which to record the muscle tracings, so slow that the ascent and descent of 

 the lever form merely one line. Let the primary current be broken at regular 

 intervals by means of a revolving drum with a platinum style attached to its 

 spindle, to make and break the primary current at every revolution (fig. 148). 

 In this way a curve such as fig. 148 is obtained. 



