Contraction of muscle, moving something, impresses one as a positive phenomenon. Relaxation seems negative—the undoing 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 antagonism 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 maintenance 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.