"I shall offer what I believe to be an original and satisfactory explanation of the beauty of form, on principles purely physiological. It is based on the proposition, that the action of every muscle is attended with a sensation which, is at first agreeable, but which, if the action is continued for a short time with intensity, and without intermission, becomes painful. That there is pleasure attending those varied motions which depend upon the actions of different muscles in succession after intervals of rest in each, we know from our own consciousness as well as from that instinctive propensity to play which we observe in children and young animals. That the prolonged action of a muscle is painful, we may readily convince ourselves by endeavoring to hold the arm for some time at right angles with the erect trunk. With the arm in this position, a pound weight on the hand or even the weight of the arm itself becomes in a few minutes almost insupportable. We presently begin to feel pain in the shoulder and anterior part of the arm, the former from fatigue of those muscles which originate from the scapula and keep the os humeri elevated, and the latter from fatigue of the muscles which originate from the scapula and os humeri, whose muscular fibres are in front of the os humeri and by their contraction elevate the fore-arm in consequence of their tendinous attachment to its bones. Yet a man may labor all day with his arms without this painful sensation; because a muscle requires but a momentary rest, in order to regain that degree of energy which is momentarily lost by action. "None but an anatomist can duly appreciate the variety of separate actions, on which depend the motions of a single limb, and the consequently numerous opportunities of rest which the muscles enjoy. To the superficial and unscientific observer, an arm is an arm; it is a single member which may be fatigued by a day's work and recruited by a night's rest. But to the anatomist the arm is a complex object, and its muscular energy is that of its component muscles, each of which may be fatigued by a minute's action and recruited by a minute's repose. It would be easy to extend this farther, and state reasons for believing that the component fasciculi and fibres of an individual muscle act still more transiently, and that their momentary and successive actions constitute the action of a single muscle.