Because the lungs and the skin together discharge the chief proportion of the watery fluid of the body. _When the skin's action is checked, the lungs have to throw off a much greater amount of fluid._ The lungs, therefore, become _over worked_, and inflammatory action sets in. 1015. _Why does cleanliness promote health?_ Because every atom of dirt which lodges upon the surface of the body serves to clog and check the working of those _minute pores_, by which much of the fluid of the body is changed and purified. In the internal parts of the system, the Creator has made ample provision for cleanliness. Every organ is so constituted that it cleanses and lubricates itself. Every surface of the inner body is perfectly clean, and as soft as silk. Nature leaves to man the care of those surfaces which are under his immediate observation and controul; and he who, from idleness, or indifference to nature's laws, is guilty of personal neglect, _opposes the evident intentions of the Creator_, and must sooner or later pay the penalty of disobedience. 1016. _Why does exercise promote health?_ Because it _assists all the functions upon which life depend_. It quickens the circulation, and thereby nourishes every part of the body, causing the bones to become firm, and the muscles to become full and healthy. It promotes breathing, by which oxygen is taken into the system, and carbon thrown off, and thereby it produces a higher degree of organic life and strength than would otherwise exist. It promotes perspiration, by which, through the millions of pores of the skin, much of the fluid of the body is changed and purified. And it induces that genial and diffused warmth, which is one of the chief conditions of a high degree of vitality. [Verse: "Love not sleep lest thou come to poverty: open thine eyes, and thou shalt be satisfied with bread."--PROV. XX.] 1017. _Why do we feel fatigue?_ Because those organs which stimulate the mechanism of the body to act, _themselves require rest and repair_. When the brain and nerves arrive at that state, they make their condition known to the system generally, by indications which we denominate _fatigue_. 1018. _Why, after rest, do we return invigorated to our labours?_ Because the nervous system has accumulated, during the hours of rest, a fresh amount of that _vital force_ which we call the nervous fluid, and by which the various organs of the body are excited to perform the duties assigned to them. 1019. _What is sleep?_ Sleep is understood to be that state of the body in which _the relation of the brain to some parts of the body is temporarily suspended_. There are some parts of the body that _never sleep_: such are the heart, the lungs, the organs of circulation, and those parts of the nervous system that direct their operations.