=Tone.=—Hitherto we have spoken of quiescence and activity, as if muscle were doing nothing when not visibly contracting. A wrong impression may be engendered by these terms. Muscle is never idle. During sleep, and still more when a person is under the influence of anæsthetics, the muscles approach the condition of machines at rest. But again the language of the workshop is inapplicable. When a headless frog is hanging from a hook its legs are slightly bent. All its muscles are weakly contracted, if we understand by contraction a condition in which the length of muscle is less than it would be were it not alive. But the flexors are tenser than the extensors, hence the crooking of hip, knee, and ankle. If the sensory 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 direction. 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 extended. 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