

MUSCLE CONTRACTION. 459 



is over before the contraction begins, for the muscle requires a 

 certain time, called the latent period, before it commences to 

 contract. 



The origin of the electric currents of muscle will be discussed 

 with nerve currents, to which the reader is referred (p. 506). 



Temperature Change. Long since it was observed in the 

 human subject that the temperature of muscles rose during their 

 activity. In frogs' muscle, a contraction lasting three minutes 

 caused an elevation of .18 C. And a single contraction is said 

 to produce a rise varying from .001 to .005 C., according to 

 circumstances. 



The production of heat is in proportion to the tension of the 

 muscle. When the muscles are prevented from shortening, a 

 greater amount of heat is said to be produced. 



The amount of heat has also a definite relation to the work 

 performed. Up to a certain point, the greater the load a muscle 

 has to move, the greater the heat produced; when this maximum 

 is reached, any further increase of the weight causes a falling off 

 in the heat production. Repeated single contractions are said 

 to produce more heat than tetanus kept up for a corresponding 

 time. 



The fatigue which follows prolonged activity is accompanied 

 by a diminution in the temperature elevation. 



MUSCLE CONTRACTION. 



Change in Form. The most obvious change a muscle under- 

 goes in passing into the active state is its alteration in shape. It 

 becomes shorter and thicker. The actual amount of shortening 

 varies according to circumstances, (a) A muscle on the stretch 

 when stimulated will shorten more in proportion than one whose 

 elasticity is not called into play before contraction, so that a 

 weighted muscle shortens more than an unweighted one with the 

 same stimulus, (b) The fresher and more irritable a muscle is, 

 the shorter it will become in response to a given stimulus ; and, 

 conversely, a muscle which has been some time removed from the 

 body, or is fatigued by prolonged activity, will contract propor- 

 tionately less, (c) Within certain limits, the stronger the stimulus 



