16 PHYSIOLOGY CHAP. 



the shortening is greater when the weight and the initial resist- 

 ance are increased. This paradoxical phenomenon is a specific 

 property of the substance of living muscle, and shows that the 

 sudden pull of the muscle and increase of tension during shortening 

 act as a stimulus on the contractile substance, and increases the 

 effect of the electrical stimulation. 



With the isometric method the tension of the muscle pre- 

 vented from shortening is far greater in the excited than in the 

 resting state. Comparison of the curves of isotonic and isometric 

 contraction, obtained from the same muscle under uniform con- 

 ditions of stimulation, show that the two curves differ very little 

 at medium temperature. When, on the contrary, the temperature 

 of the atmosphere is lowered to about 5 C. the two tracings 



present distinctive char- 

 acters. Fig. 10 plainly 

 shows that the isometric 

 curve reaches its maximum 

 more rapidly than the iso- 

 tonic curve, and that in 

 the former maximal tension 

 persists for a certain time, 

 while in the second it passes 

 suddenly from the height of 

 the contraction phase to the 



FIG. 10. Comparison of isotonic (a) and isometric (/-) phase of relaxation. 



inyograms from the same muscle. (Gad.) The eor.iv.o-nn /"I QOJA cfnrliprl 



isometric curve is reversed because in Gad's myo- \J.yw) &LUI 



graph the lever is pulled down instead of up by foe influence of the load 

 increasing tension of the muscle. 



upon isometric curves by 



submitting the muscle to sudden changes of tension during its 

 contraction. Such changes, whether a temporary or permanent 

 increase or decrease, always induce marked diminution of tension 

 in the muscle in a degree which depends not on the magnitude, 

 but on the abruptness of the change, and is more pronounced 

 the later the alteration in tension occurs in the contraction. 

 In the body these conditions of isotonia and isometria are, of 

 course, seldom realised. A certain amount of contraction is nearly 

 always needed to overcome the resistance that diminishes or 

 increases during the course of excitation. The muscles, in other 

 words, are almost always employed in carrying out an external 

 mechanical task under various conditions, which differ from the 

 experimental conditions of isotonia and isometria. The isometric 

 method is an analytic means of eliminating the complications of 

 changes of form and internal friction, so as to obtain the simpler 

 curve of the changes of tension or of longitudinal molecular attrac- 

 tion, which are the fundamental effects of muscular excitation. 



III. The activity of skeletal muscle in the body differs in 

 another respect from that above described. Under natural 



