3. The plastic elements of the blood are enabled to repair the materials removed from the organs during the combustions of work.

4. Finally, time is given to the muscular and nervous elements for making a new provision of energy by a physiological mechanism which is still unknown.

Repose is the condition which is diametrically opposed to work, and the phenomena observed in these two so different states are absolutely inverse. Muscular work causes exaggeration of vital phenomena, and gives to all the functions a greater intensity: it quickens the pulse and respiration and raises the temperature of the body. Rest slows the pulse and respiration and lowers the temperature.

Like work, repose has its degrees, and these degrees are very relative. In a practised runner, to walk for a time is to rest; in the sick man, used to lie on his back, to sit upright is work.

Sleep is complete repose, because in this condition all the muscles of animal life are relaxed, and those of organic life work with less energy. The respiration and pulse are less frequent, the temperature lower than in the waking state. Further, an organ which works without ceasing when we are awake, the brain, rests during sleep, and the circulation in it is less active, as has actually been observed in men with holes in the skull.

The fall of temperature during sleep is a proof of diminished combustion, and of minimal formation of waste products. Further, it has been observed that only half as much carbonic acid is eliminated in the sleeping as in the waking state.

The fatigue produced by continuous work is intense in proportion to the expenditure of force. A violent effort cannot be long sustained; but if the most violent exercise is interrupted by periods of rest which, though very short, are sufficiently frequent, the exercise may be carried on for a very long time.

In prize-fights, the fight is stopped every two or three minutes, and two minutes' rest is taken. This interruption of the fight at short intervals would seem at first to lessen its brutality; but it is really a way of rendering its results more murderous. Formerly, when the rounds were longer, lasting ten minutes, the boxers quickly became fatigued. Their blows became less certain, and produced less serious injuries. Weariness, quite as much as wounds, made it impossible to continue the fight. Now-a-days, with short rounds and frequent rests, the adversaries husband their strength, and their blows are as hard at the end as they were at first. The beaten man has to yield, not because he is wearied, but because he is seriously injured. In spite of their strength and their wonderful staying powers, the combatants could not, without these periods of rest, endure the prolonged fatigues of these fights, which often last several hours.