THE PHENOMENON OF CONTRACTION. 27 



bands is imbibed into the substance composing the dim bands. 

 The process is evidently a reversible one; during the phase of 

 relaxation the absorbed or imbibed material passes back to the 

 light band. 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 fingers 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 then 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 



