6 COMPARATIVE PHYSIOLOGY 



contracting through the second step the work done is t^l^l. 

 The total work is the sum of a series of products repre- 

 sented by the rectangular areas which are together equivalent 

 to the area T,EC. 



If TjC is curvilinear, the area enclosed by the curve may be 

 made as near as we like to the sum of these rectangles by 

 making A^ sufficiently small. 



To construct such a tension-length curve the muscle is 

 excited isometrically, i.e. an arrangement is used by which the 

 tension is recorded by a spring lever without appreciable 



Fig. I. 



movement of the muscle itself. In general the records so 

 obtained resemble those of isotonic twitches. The procedure 

 adopted is as follows. The muscle is held rigidly by a clamp 

 which can be screwed up and down a graduated scale. At 

 the beginning of the experiment the clamp and muscle are 

 screwed down until the cord attaching the other extremity of 

 the latter to the tension lever is just tight, but not sufficiently 

 so to stretch the muscle appreciably. A stimulus is now given, 

 and the tension recorded. The screw of the clamp is now 

 turned so as to raise it i mm., thereby slackening the cord. 



