MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 749 



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. 255). 

 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. 256). 



(c) Influence of Previous Stimulation Fatigue. 

 If a muscle is stimulated by a series of equal 

 shocks thrown in at regular intervals, and the 

 contractions recorded, it is seen that at first 

 each curve overtops its prede- 

 cessor by a small amount.* This 

 phenomenon, which is regularly 

 _ observed in fresh skeletal muscle 



Fig. 256. 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. 



(Fig. 260), although it was at one time supposed to be peculiarly a 

 property of the muscle of the heart (Fig. 261), 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. 258, 259, 288). 



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, 



* Guthrie has recently described an interesting series of phenomena illus- 

 trating the influence of previous stimulation on the irritability of muscle. 

 Under given conditions, for example, strong stimulation increases the response 



of skeletal muscles to subsequent single stimuli, it may be, by hundreds per 



cent. 



