The heart, though making contractions at the rate of seventy-two beats a minute, is able to continue its work throughout the life of an individual, since each contraction of this muscle is followed by an interval of rest, during which the cells recuperate. Stimulate the heart beats beyond the normal rate, and a point is soon reached at which poisonous products are not replaced by fresh cells, since the intervals of rest are insufficient. Similar conditions are met in the action of the diaphragm and of the chest and abdominal muscles used in respiration.
Just here a distinction in function needs notation :
The muscles that move involuntarily, those that are not subject to the human will, never know absolute rest, for they continue their labors whether the body be asleep or awake.
On the other hand, those muscles, the action of which depends upon the direction of the human will, cannot work continuously, lest fatigue with fatal exhaustion follow.
Seemingly, automatic labor, labor not directed by the highest function of consciousness, does not wear. It is only conscious work that requires for recuperation and muscle-rebuilding other means than simple non-use or
