MUSCLE. 23 



When a weight is attached to the lever the height of the contrac- 

 tion diminishes as the weight is increased, until, finally, the muscle 

 fails to raise the weight at all. The product of the weight raised, 

 and the height through which it is lifted, represents the work done by 

 the muscle ; up to a certain point, an increase in the weight raised 

 increases the work done by the muscle. 



Contracting muscle differs from resting muscle in being more 

 extensible and more elastic, that is, it returns to its original length 

 more rapidly than resting muscle when the force stretching it is 

 removed. 



Isotonic and Isometric Curves. When a muscle lifts a weight 

 attached to a lever, the weight follows the movement of the lever, 

 and the pull or tension which it exerts upon the muscle remains un- 

 changed during the contraction ; the curve thus obtained is called 

 isotonic. The muscle may be made, however, to pull against a strong 

 spring so that its length remains almost unaltered during its contrac- 

 tion, although it exerts a varying tension upon the spring ; the minute 

 movements of the spring are magnified and recorded photographically. 

 In this case the contractile stress set up in the muscle varies 

 throughout the contraction, though the length of the muscle undergoes 

 no appreciable change. Curves thus obtained are called isometric, and 

 resemble isotonic curves in their general form. 



Tetanus. If a second stimulus is sent into a muscle before the 

 shortening caused by the first stimulus is at an end, the muscle 

 shortens still further, this being called summation of effects. When 

 the stimuli are rapidly repeated (fifty or more per second), the muscle 

 contracts very strongly, and remains contracted so long as the stimuli 

 are continued ; the prolonged contraction is known as tetanus. 



Constant Current. When a constant current is passed through a 

 muscle, it enters at one point called the anode, and, after traversing 

 the muscle, leaves it at the kathode. When the circuit is completed, 

 i.e. at the make of the current, the muscle gives a single twitch, and 

 there is another twitch when the current is broken; during the 

 passage of the current the muscle remains relaxed. The contraction 

 taking place when the current is made starts at the kathode, whereas 

 the contraction occurring at the break of the current starts at the 

 anode. This can be observed by clamping a skeletal muscle in the 

 middle and attaching a lever to each end ; one electrode is placed on 

 each half of the muscle. When the current is made, the portion of 

 the muscle connected with the kathode contracts first, and the lever 

 attached to this half of the muscle rises before the other lever. The 

 same phenomenon is still more readily seen, and can be directly 



