MUSCLE-NERVE. 



is called isotonic. When the shortening of the muscle is prevented 

 by a constantly increasing resistance, so that all its power is used 

 in overcoming the resistance, the contraction is called isometric. 



The resistance is usually obtained by the use of a spring to which 

 the muscle is attached, the energy of the muscle being stored in the 

 spring in the form of tension, to be liberated as heat as soon as the 

 muscle relaxes. 



1. Graduation of the Isometric Spring. In order to estimate 

 the isometric value of a muscle contracting against resistance of- 

 fered by a spring, it is necessary to interpret the spring's resistance 

 in terms of weight. Reverse the spring of the heavy myograph 

 (Porter's), attaching its hook to the scale pan beneath. Bring the 

 writing point of the lever against the smoked paper of a drum ar- 

 ranged to be moved by hand. Revolve the drum a half turn to re- 

 cord a base line. Place a loo-gram weight in the scale pan. The 

 spring will be bent to a certain extent and the lever will mark a 

 descending line on the drum. Move the drum, slightly, to record 

 the lower limit of the spring's bend. Repeat with a 2oo-gram 

 weight, and so on up to 800 grams. 



2. Make a gastrocnemius-sciatic preparation. Attach the tendo 

 Achillis to the isometric spring. Adjust the writing point of the 

 lever against the smoked paper of a rapidly revolving drum. Stim- 

 ulate the nerve with a maximal break shock from an inductorium. 

 An isometric curve will thus be obtained. 



3. Release the muscle from the spring and attach it to the ordi- 

 nary writing lever weighted with 20 grams. The lever, in this case, 

 should be as long as that used for recording the isometric curve. 

 With the drum revolving at the same rate as before, stimulate the 

 nerve so as to record a twitch, as nearly as possible, under the re- 

 corded isometric curve. 



4. Compare the two curves (i) and (2), as to form and as to 

 work done. To find the amount of tension overcome, as indicated 

 by weight, compare the height of the isometric curve with the de- 

 pression of the spring in (i). 



What influence does tension have on muscle work? How does 



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