140 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



hence does no external work. Although no external work is accomplished 

 under these circumstances, internal work is being done, as is evidenced by 

 the heat evolved by the muscle and the fatigue produced. Unquestionably 

 mechanical energy is developed within the muscle in all these cases, but it 

 is all converted to heat before it leaves the muscle. 



The amount of weight is an important factor in determining the extent to 

 which a muscle will shorten when excited by a given stimulus, and, therefore, 

 the quantity of work which it will accomplish. If a muscle be after-loaded, 

 i. e. if the weight be supported at the normal resting length of the muscle, and 

 the muscle be excited to a series of maximal contractions, the weight being in- 

 creased to a like amount before each of the succeeding excitations, there is, in 

 general, a gradual lessening in the height of the contractions, but the de- 

 crease in height is not proportional to the increase of the weight. The 

 decrease in the height of contractions is, as a rule, more rapid at the beginning 

 of the series than later, though at times an opposite tendency may show itself 

 and the increasing weights temporarily increase the irritability and therefore 

 increase the amount of shortening. The effect of tension to increase the activ- 

 ity of the contraction process is seen if a muscle which is connected with a 

 strong spring or heavy weight be excited to isometric contractions and in 

 the midst of a contraction be suddenly released ; the muscle under such cir- 

 cumstances is found to contract higher than when excited by the same stimulus 

 without being subjected to tension. 1 The effect of tension on the activity of 

 muscular contractions is to be clearly seen iu the case of the heart muscle. 

 A l-ist' of pressure of the fluid within the isolated heart of a frog increases 

 the strength as well as the rate of the beat. 



If the weight be gradually increased, although the height of the contrac- 

 tions is lessened, the work will for a time increase, and a curve of work (con- 

 structed by raising ordinates of a length corresponding to the work done, 

 from points on an abscissa at distances proportional to the weights em- 

 ployed), will be seen to rise. After the weight has been increased to a cer- 

 tain amount the decline in the height of contractions will be so great that the 

 product of the weight into the height will begin to decrease, and the curve of 

 work will fall, until finally a weight will be reached which the contracting 

 muscle can just support at, but not raise above, its normal resting length 

 This weight will be a measure of the absolute muscular force. 



Example. 



Load Height of lift Work 



(grams). (millimeters). (grammillimeters). 



13 



30 11 330 



60 9 540 



90 7 630 



120 5 600 



150 3 450 



180 2 '. . 360 



210 



1 Fick : Mechanische A rbeit, etc., S. 132. Santesson : Skandinavisches Archiv fiir Physiologic, 

 1889, i. S. 56. 



