THE MUSCULAR SENSE.--When we lift an object, we feel a sensation of weight, which we can compare with that experienced in lifting another body. [Footnote: If a small ivory ball be allowed to roll down the cheek toward the lips, it will appear to increase in weight. In general, the more sensitive parts of the body recognize smaller differences in weight, and the right hand is more accurate than the left. We are very apt, however, to judge of the weight of a body from previous conceptions. Thus, shortly after Sir Humphrey Davy discovered the metal potassium, he placed a piece of it in the hand of Dr. Pierson, who exclaimed: "Bless me! How heavy it is!" Really, however, potassium is so light that it will float on water like cork.] By balancing it in the hand. The muscular sense is useful to us in many ways. It guides us in standing or moving. We gratify it when we walk erect and with an elastic step, and by dancing, jumping, skating, and gymnastic exercises. NECESSITY OF EXERCISE.--The effect of exercise upon a muscle is very marked. [Footnote: The greater size of the breast (pectoral muscle) of a pigeon, as compared with that of a duck, shows how muscle increases with use. The breast of a chicken is white because it is not used for flight, and therefore gets little blood.] By use it grows larger, and becomes hard, compact, and darker-colored; by disuse it decreases in size, and becomes soft, flabby, and pale. Violent exercise, however, is injurious, since we then tear down faster than nature can build up. Feats of strength are not only hurtful, but dangerous. Often the muscles are strained or ruptured, and blood vessels burst in the effort to outdo one's companions. [Footnote: Instances have been known of children falling dead from having carried to excess so pleasant and healthful an amusement as jumping the rope, and of persons rupturing the Tendon of Achilles in dancing. The competitive lifting of heavy weights is unwise, sometimes fatal.] (See p. 278.) Two thousand years ago, Isocrates, the Greek rhetorician, said: "Exercise for health, not for strength." The cultivation of muscle for its own sake is a return to barbarism, while it enfeebles the mind, and ultimately the body. The ancient gymnasts are said to have become prematurely old, and the trained performers of our own day soon suffer from the strain they put upon their muscular system. Few men have sufficient vigor to become both athletes and scholars. Exercise should, therefore, merely supplement the deficiency of our usual employment. _A sedentary life needs daily, moderate exercise, which always stops short of fatigue_. This is a law of health. (See p. 280.)