GENERAL PHYSIOLOGY OF MUSCLE AND NERVE TISSUE 55 



electromagnet and tracing lever or through a tracing lever attached 

 to the key of the primary circuit. There will thus be three tracings 

 upon the drum: (a) the myogram; (b) the time tracing; (c) the 

 stimulus tracing, the latter showing the time when the stimulus is 

 made. Note that the myograph lever does not rise until a certain 

 time after the stimulus is given. This period is the latent period. 

 ^Yhat is the length of the latent period? 



(3) Trace another myogram, but as the lever is sinking back 

 toward the abscissa stimulate a second time. Note that the result 

 is a double-crested myogram and that the second is higher than 

 the first. 



(4) Trace another myogram resulting from a series of stimuli 

 occurring, in rapid succession, if possible about ten times per second. 

 Note that the result is a myogram with a series of crests and that 

 the lever does not fall back to the abscissa between the successive 

 stimuli. Note that the first few crests are progressively higher and 

 higher. This phenomenon is called the "stair-case series of con- 

 tractions," and is usually observed when a muscle is given a series 

 of stimuli after a period of rest. 



(5) Vary the above experiment by increasing the rapidity of 

 stimuli to 20 per second. This may be done through the use of 

 a toothed wheel as a key in the primary circuit, or through modifica- 

 tion of the Neef hammer, which causes it to vibrate slowly. Use 

 medium speed of kymograph. Note that the result is a myogram 

 with a serrated crest, the serrations indicating the result of the 

 several stimuli. 



(6) Stimulate with a series of induct shocks caused by the rapid 

 making or breaking of the primary circuit through the vibration 

 of the Neef hammer. Use medium-speed drum. Note that this 

 throws the muscle into a condition of typical tetanus. 



Trace a series of tetanus curves, each lasting about three or four 

 seconds. 



XIII. THE WORK DONE BY A MUSCLE. 



(a) To determine the amount of work done by a single contraction. 

 (b) To determine the total amount of work done by a muscle, (c) 

 Reaction changes in fatigued muscles. 



1. Appliances. Same as Lesson XII.; also 50-gram weight and 

 20-gram or 30-gram weight. 



2. Preparation. Arrange electric apparatus for a series of 

 break-induction shocks. 



3. Operation. Make and mount a gastrocnemius preparation 

 for indirect stimulation. 



4. Observations. Upon a slow drum record' in close order a 

 series of break contractions. 



