28 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



exhibit corresponding differences. As we have seen, the appearance 

 of the muscle fiber when viewed by polarized light indicates that 

 during the phase of shortening the most marked physical change 

 occurs in the anisotropic band. Whatever may be the nature 

 of this change, it is evidently a reversible one. After reaching 

 its maximum it proceeds in the opposite direction, the particles 

 return to their original position, and a relaxation occurs. Many 

 conditions, some of which will be described below, alter the time 

 necessary for these processes, that is, the duration of the simple 

 contraction. It is noteworthy that it is the phase of relaxation 

 which may be most easily prolonged or shortened by varying 

 conditions. 



Isotonic and .Isometric Contractions. In the method of recording the 

 shortening of the muscle that is described above the muscle is supposed to con- 

 tract against a constant load which it can lift. Such a contraction is spoken 

 of as an isotonic contraction. If the muscle is allowed to contract against 

 a tension too great for it to overcome a stiff spring, for instance it is prac- 

 tically prevented from shortening, and a contraction of this kind, in which 

 the length of the muscle remains unchanged, is spoken of as an isometric 

 contraction. A curve of such a contraction may be obtained by magnifying 

 greatly, by means of levers, the slight change in the stiff spring against which 

 the muscle is contracting. Such a curve gives a picture of the liberation of 

 energy within the muscle during contraction. 



The usual oval form of dynamometer employed to record the grip of the 

 flexors of the ringers gives an isometric record of the energy of contraction 

 of these muscles. 



Fig. 9. Effect of varying the strength of stimulus. The figure shows the effect upon 

 the gastrocnemius muscle of a frog of gradually increasing the stimulus (breaking induction 

 shock) until maximum contractions were obtained. The stimuli were the.n decreased in 

 strength and the contractions fell off through a series of gradually decreasing submaximal 

 contractions. The series up and down is not absolutely regular owing to the difficulty of 

 obtaining a regular increase or decrease in the stimulus. (The prolongations of the 

 curves below the base line are due to the elastic extension of the muscle by the weight dur- 

 ing relaxation.) 



Effect of Strength of Stimulus upon the Simple Contraction. 

 The strength of electrical stimuli can be varied conveniently and 



