MUSCLE 



645 



traction. Many circumstances affect the form of the muscle curve 

 and its time-relations. I 



(a) Influence of the Load. The first effect of contraction is to 

 suddenly stretch the muscle, and the more the muscle is loaded the 

 greater will this elongation be. So that at the beginning of the 

 actual shortening part of the energy of contraction is already 



FIG. 230. CURVE OF A SINGLE MUSCULAR CONTRACTION OR TWITCH TAKEN 

 ON SMOKED GLASS WITH SPRING MYOGRAPH AND PHOTOGRAPHED. 



Vertical line A marks the point at which the muscle was stimulated ; time 

 tracing shows T J^ of a second (reduced). 



expended without visible effect, and has to be recovered from the 

 elastic reaction during the ascent of the lever. 



Then the inertia of the lever itself and of its load comes into play, 

 and may carry the curve too high during the up-stroke and too low 

 during the down-stroke. This can be minimized by making the 

 lever very light, and attaching the weight close 

 to the fulcrum, so that it has only a small range 

 of movement, and never acquires more than a 

 small velocity. The contraction of a muscle 

 loaded by a weight which is not increased or 

 diminished during the contraction is said to 

 be iso-tonic, for here the tension of the muscle 

 is the same throughout, and its length alters. 

 When the muscle is attached very near th2 ful- 

 crum of the lever, so that it acts upon a short 

 arm, while the long arm carrying the writing- 

 point is prevented from moving much by a 

 spring, the muscle can only shorten itself very 

 slightly ; but the changes of tension in it will 

 be related to those in the spring, and therefore 

 to the curve traced by the writing-point. Such 

 a curve is called iso-metric, since the length 

 of the muscle remains almost unaltered. In 

 the body muscles usually contract under con- 

 ditions more nearly allied to those of the iso- 

 metric than to those of the iso-tonic contraction. 



The maximum of the iso-metric curve (the 

 maximum tension with practically constant 

 length) is sooner reached than that of the iso- 

 tonic (the maximum contraction with constant tension). From 

 this it has been concluded that as the muscle shortens its coefficient 

 of elasticity continuously diminishes (Pick), or, what comes to the 

 same thing, its extensibility continuously increases. It follows that 

 the tension of a muscle contracting against resistance, especially at 

 the beginning of its contraction, is greater than would be the case 

 when the muscle, contracting isotonically, had attained the same 

 length. 



FIG. 231. CONTRAC- 

 TIONS OF SMOOTH 

 MUSCLE: CAT'S 

 BLADDER (C. C. 

 STEWART). 



Stimulated with pro- 

 gressively stronger in- 

 duction shocks. The 

 lowest line is the time 

 trace (lo-second inter- 

 vals). Immediately be- 

 low the muscular con- 

 tractions are marked 

 the points at which the 

 stimuli were thrown in. 



