MUSCLE 



647 



lift, and the work falls off till ultimately the muscle is unable to 

 raise the weight at all. 



The manner of application of the weight has an influence on the 

 work done by the muscle. If it is applied before the contraction 

 begins, so that the muscle is already stretched at the moment of 

 stimulation, a cause of error and uncertainty is introduced ; for it is 

 known that mere stretching of muscle affects its metabolism, and 

 therefore its functional power. So that it is usual in experiments 

 of this kind to after-load the muscle -that is, to support the lever 

 and its load in such a way that the weight does not come upon the 

 muscle till contraction has just begun. The ' absolute contractile 

 force ' of an active muscle may be measured 

 on this principle by determining the weight 

 which, brought to bear upon the muscle at the 

 instant of contraction, is just able to prevent 

 shortening without stretching the muscle. It, 

 of course, depends, among other things, on the 

 cross-section of the muscle. During the con- 

 traction the absolute force diminishes continu- 

 ally, 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 con- 

 traction the absolute force is zero. Hence a 

 muscle works under the most favourable con- 

 ditions, when the weight decreases as it is 



FIG. 234. INFLUENCE OF TEMPERATURE ON THE SMOOTH MUSCLE CURVE 

 CAT'S BLADDER (C. C. STEWART). 



Contractions at different temperatures with the same strength of stimulus. 

 The temperatures (Centigrade) are marked on the curves. 



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 hori- 

 zontal, a weight in the hand is felt less and less as it is raised, since 

 its moment, which is proportional to its distance from a vertical line 

 drawn through the lower end of the humerus, continually diminishes. 

 (b) Influence of Temperature on the Muscular Contraction. 

 Increase of temperature of the muscle up to a certain limit dimin- 

 ishes the latent period and the length of the curve, and increases the 

 height of the contraction, but beyond this limit the contractions are 

 lessened in height (Fig. 233). 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 start from, and 

 moderate cooling may increase the height of the curve. In the heart 



