The muscle of the body is that fleshy part, with which every one is familiar. It consists of fibres which lie parallel to each other. This fibrous, or filamentous part, has a living endowment, a power of contraction and relaxation, termed irritability. A single muscle is formed of some millions of these fibres combined together, having the same point of attachment or origin, and concentrating in a rope or tendon, which is fixed to a moveable part, called its insertion. We may demonstrate upwards of fifty muscles of the arm and hand, all of which must consent to the simplest action; but this gives an imperfect view of the extent of the relation of parts which is necessary to every act of volition. We are most sensible of this combination in the muscles, when inflammation has seized any of the great joints of the body : for even when in bed, every motion of an extremity gives pain, through the necessity of a corresponding movement in the trunk. When we stand, we cannot raise or extend the arm without a new position of the body, and a poising of it, through the action of a hundred muscles.

ON THE ACTION OF THE MUSCLES OF THE ARM.

We shall consider this subject under two heads ; in the first, we shall give examples of the living property of the muscles ; and then of the mechanical contrivances, in their form and application.