VOLUNTARY MUSCLE. 35 



Repeat the experiment using the gastrocnemius of the left 

 leg, the circulation of which is not disturbed. Compare Treppe, 

 Fatigue, Contracture and Rest in the circulated and non- 

 circulated muscle. The circulated muscle may reach a point 

 beyond which fatigue fails to develop further. This is the 

 fatigue level. Explain why this occurs. Alter the rate of 

 stimulation and explain its effect on the height of the fatigue 

 level. 



FIG. 10. Graphic record of two voluntary contractions showing their tetanic nature. Made 

 by pull of the index finger against a stiff spring at the end of which a writing point is attached. 



TETANUS. 



Voluntary muscular action is caused by a rapid succession of 

 impulses travelling outward along a motor nerve. The muscle 

 therefore does not respond by a simple twitch, but by a more or less 

 sustained contraction. Even the quickest voluntary movements 

 are a fusion of several simple contractions. 



Several proofs have been obtained of the composite nature of 

 such contraction. First it has been found by the use of a sensitive 

 galvanometer that several impulses pass along a nerve which is 

 exciting a muscle even to brief contraction. 



Second, a sound can be heard through a stethoscope applied 

 to a muscle contracting voluntarily. This sound is no doubt due 



