There are many reasons why the numerous and bulky muscles with which the body is endowed must be constantly used. In the third essay in this volume we saw that the flow of the lymph and the blood in the veins is largely dependent upon the movements of the limbs. Muscular exercise, therefore, must be taken to prevent the circulating streams from growing sluggish. Obviously, many evils must arise if they do so. Not only would the muscles be starved by the slowness with which they received their food, but they would also be poisoned by the slowness with which the products of their own metabolism were removed. The blood-stream would become tainted, and the brain, which requires pure blood, would suffer. In other ways, however, than by their action on the vessels themselves do the muscles help the circulation. Exercise has secondary effects upon the circulatory and respiratory centres in the base of the brain, and makes the heart beat more strongly and the diaphragm contract more forcibly. The influence of the latter upon the circulation we have already described; but its vigorous action is required not only to aid the circulation through the liver and viscera, but to inflate the lungs to their fullest extent. In the breathing of a man who takes no exercise only a small part of the air which the lungs hold is pumped out at each breath, the greater part remaining stagnant in chambers which are practically unused. Thus, not only is the revenue of oxygen diminished, but there are numerous little crannies in the body filled with still, warm air which are ideal nurseries for bacteria. The devil of consumption does not allow such dwellings to long remain swept and garnished. Without exercise the sweat glands of the skin will not act, and its pores get closed up. The muscular coats of the alimentary canal, too, reflect in their state of health the condition of the voluntary muscles; laziness is followed by constipation, for if the voluntary muscles rust, so do the involuntary. The muscles, moreover, must be well developed and kept in a healthy condition for their own sake. They form a large part of the whole bulk, and no healthy man can have unhealthy muscles. Ignorant people sneeringly say that they have no ambition to lift weights or bend pokers; but they should remember that they are dependent upon their muscles for their bodily warmth, and also to save their internal organs from being oppressed by their own weight.