944. Why can we throw back the fingers after they have been raised?

Because the muscles at the back of the arm, whose tendons are attached to the back of the fingers, contract and restore them to their former position.

945. What degree of strength do the muscles possess?

The degree of strength of a muscle depends upon the healthy condition of the muscle, the amount of stimulus which it receives at the time of exertion, and the manner in which its powers are applied.

The great muscle of the calf of the leg has been found, when removed from a dead body, to be capable of sustaining a weight equal to seven times the weight of the entire body.

But the contractile power of the living muscles is very great: the thigh bone has frequently been broken by muscular contractions in fits of epilepsy. And in cases where there has been a dislocation of the thigh, the head of the thigh-bone being thrown out of its socket, ([Fig. 56]) it has been found necessary to employ strong ropes, attached to a wheel turned by several hands, in order to overcome the contraction of the excited muscles, and to enable the operator to restore the bone to its place.

946. What is the stimulus which sets the muscles in action?

The muscles are excited to action by the nerves, which they receive from the spinal cord.

947. Why does it require the influence of the will to set the arms in motion?