MUSCLE 273 



roots of the sciatic nerve are cut, the leg straightens out. So 

 long as the nerve was intact the weight of the limbs acted 

 as a stimulus to sensory nerve-endings, causing a reflex 

 "tone" of the flexor muscles via the spinal cord. The 

 tone of the extensor muscles was less because they were 

 not stretched by the weight of the limbs. Every joint 

 is under the influence of antagonistic muscles which are 

 perpetually watching one another. When the limb is extended 

 the flexors become anxious. When it is flexed the extensors 

 get ready for a spring. Only when it is half flexed is there 

 anything approaching to a truce. And this in most cases is the 

 position of greatest comfort. But even when most at rest, 

 muscles still possess a certain degree of tone. The tendency to 

 shortening in one set causes it to pull against, and thereby 

 increases the tone of, its opponents. When a muscle contracts 

 it does not lift a loose bone. It has to overcome the tone of the 

 muscles which would cause a movement in the opposite direc- 

 tion. And here another adjustment comes into play. The 

 same gross stimulus which leads to the contraction of A starts 

 impulses of a finer kind for B, directing it to relax its tone. 

 We have seen how the heart and bloodvessels are under the 

 influence of two sets of nerves of opposite sign anabolic, 

 diminishing irritability ; katabolic, increasing it. All muscles 

 are under similar management ; but we can rarely detect the 

 influence of the anabolic, inhibitory nerves, the brakes, because 

 the katabolic display is overwhelmingly conspicuous. We 

 must be content with two experimental demonstrations, An 

 animal's hamstrings have been cut ; the flexor muscles of its 

 thigh are therefore severed from their attachments below the 

 knee. The tone of the extensor muscle keeps this joint ex- 

 tended. If now the pad of the foot be tickled, the flexor 

 muscles contract, just as they would do if they were still able 

 to carry out the reflex action of raising the foot. They cannot 

 do this, because their tendons are divided ; nevertheless, the 

 knee bends owing to reflex relaxation of the extensor muscles. 

 Still more striking evidence of reciprocal contraction and 

 relaxation is afforded by the claw-muscles of a crayfish. A 

 weak stimulus to its nerve causes the claw to set open; a stronger 

 stimulus causes it to close. Both these movements are due, not 

 to " contraction," but to change of tone. Under certain con- 



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