MUSCLE 



565 



stretching the muscle. It, of course, 

 depends, among other things, on the 

 cross-section of the muscle. During 

 the contraction the absolute force di- 

 minishes continually, so that a smaller 

 and smaller weight is sufficient to stop 

 any further contraction, the more the 

 muscle has already shortened before it 

 is applied. At the maximum of the 

 contraction the absolute force is zero. 

 Hence a muscle works under the most 

 favourable conditions when the weight 

 decreases as it is raised, and this is the 

 case with many of the muscles of the 

 body. During flexure of the forearm 

 on the elbow, with the upper arm 

 horizontal, a weight in the hand is felt 

 less and less as it is raised, since its 

 moment, which is proportional to its dis- 

 tance from a vertical line drawn through 

 the lower end of the humerus, continu- 

 ally diminishes. 



(b) Influence of Temperature on 

 the Muscular Contraction. Increase 

 of temperature of the muscle up 

 to a certain limit diminishes the 



latent period and the length of the 



u t u* r FIG. 172. FATIGUE CURVE OF 

 curve, and increases the height ot MUSCLE (FROG'S GASTRO- 



CNEMIUS). 



Below is shown the arrangement 

 with which the curve figured was 



is continued along the lever and 



connected with a sewing-needle, 



the p^ of wnich j us t dips into 



the contraction, but beyond this 

 limit the contractions are lessened 

 in height. Marked diminution of 

 temperature causes, in general, an 

 increase in the latent period and 

 length, and a decrease in the 

 height of the contraction. It is 

 evident that much depends upon 

 the normal temperature which we attached to the upper end of the 



r r muscle or the clamp. Or a wire 



Start from, and moderate COOling on the lever may be made to close 



may increase the height of the 

 curve. In the heart the effect of 



COld in Strengthening the beat is touches the mercury the muscle is 



stimulated automatically. 



often very marked. 



(c) Influence of Previous Stimulation. If a muscle is stimu- 



