274 THE BODY AT WORK 



ditions, a current passed through its abductor muscle, the claw 

 being open at the time, causes closure by inhibiting the tone of 

 this muscle. In this case the stimulus acts directly on the 

 muscle, producing an effect which is opposite to the one we are 

 accustomed to associate with stimulation ; in place of con- 

 traction, relaxation. 



Contraction of muscle, moving something, impresses one as 

 a positive phenomenon. Relaxation seems negative the un- 

 doing of contraction and to a very large extent this attitude 

 of mind is justified. Return of a muscle to its full length is due 

 either to stretching by the weight it has lifted, or to the an- 

 tagonism of other muscles. An isolated muscle lying on a 

 pool of quicksilver does not return to its full length after it 

 has contracted. But it is necessary to banish the machine 

 idea. A machine gives out all the energy it has in store. 

 Muscle is extremely parsimonious. No stimulus can induce 

 it to part with more than a fraction of its energy. Recovery 

 is as definite a function as disturbance. A machine starts 

 when a crank is moved, stops when it is replaced. Muscle has 

 a certain degree of automatism, although its tendency to act 

 on its own account has been almost completely transferred to 

 the governing nervous system. Muscle and nerve work 

 together, and the efficiency of muscle depends upon the main- 

 tenance of its relations with its nerve. If the nerve is cut, 

 the muscle atrophies. We will not stop to consider whether 

 wasting may be properly attributed to disuse, or to vaso- 

 motor changes. In its lowest form nervous influence shows 

 itself in the regulation of the nutrition of muscle. A somewhat 

 more forcible exhibition of control is seen in the regulation of 

 tone. The maximum is reached when a wave of undoing 

 which has passed down a nerve infects the protoplasm of 

 muscle with the same tendency to disintegration. The muscle- 

 substance explodes. The muscle shortens. 



Remarkable evidence of the existence of muscle-tone is 

 afforded by the knee-jerk. Place a person on an upright 

 chair, with his legs crossed, muscles lax, foot hanging free. 

 With a paper-knife or the end of a stethoscope, or even the 

 hand used edgewise, tap the ligament which connects his knee- 

 cap with his shin. The tap is instantly followed by a jerking 

 forward of the foot. The deep muscles of the thigh, vastus, 



