MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 723 



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 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 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 diminishes 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. 248). Marked diminution of temperature 

 causes, in general, an increase in the latent period 

 and length, and a decrease in the height of the 

 contraction. In the heart the effect of cold in 

 strengthening the beat is often very marked. 

 Temperature affects the contraction curve of 

 smooth muscle much in the same way as that 

 of striated muscle (Fig. 249). 



(c) Influence of Previous Stimulation Fatigue. 

 If a muscle is stimulated by a series of 

 equal shocks thrown in at regular intervals, 



Fig. 249. 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. 



and the contractions recorded, it is seen that at first each curve 

 overtops its predecessor by a small amount. This phenomenon, 

 which is regularly observed in fresh skeletal muscle (Fig. 253), 

 although it was at one time supposed to be peculiarly a property 

 of the muscle of the heart (Fig. 254), is called the ' staircase,' and 

 seems to indicate that within limits the muscle is benefited by 

 contraction and its excitability increased for a new stimulus. Soon, 

 however, in an isolated preparation, the contractions begin to decline 

 in height, till the muscle is at length utterly exhausted, and reacts 

 no longer to even the strongest stimulation (Figs. 251, 252, 279). 



A conspicuous feature of the contraction-curves of fatigued 

 muscle is the progressive lengthening, which is much more marked 

 in the descending than in the ascending periods; in other words, 



