THE CONTRACTION OF MUSCLE 



541 



the weight be so arranged that the muscle only raises it during its 

 contraction, it is then said to be " after- loaded." When the resistance 

 is slight, so that the muscle can change its shape during contraction 



FIG. 275. A SINGLE MUSCULAR CONTRACTION (FROG'S GASTROCNEMIUS). 



From 1 to 2 is the latent period ; from 2 to 3, the period of shortening ; from 3 to 4, 

 the period of relaxation. Time in -, fa seconds. 



the curve yielded is said to be " isotonic." Its length alters, but the 

 tension remains equal throughout the contraction. This is the case 

 when the contraction is registered by such forms of apparatus as 



FIG. 27(5. COMPARISON OF CONTRACTIONS OK RED AND WHITE MUSCLE OF RABBIT, 



STIMULATED INDIRECTLY. (M. S. Pcmbrey.) 



Upper curve is response oi' the red solcus, and lower curve that of the white gastroc- 

 nemius. Time marker, 50 per second. The tracing to be read from right to 

 left. 



are shown in Figs. 272, 273. When the muscle contracts against a 

 large resistance, so that it can shorten but little, the recording lever 

 gives an " isometric " curve. The muscle in these conditions remains 

 of approximately the same length, but alters in tension. 



