Reverence for this unity of life increases the more carefully this strange fact, called the human body, is studied, the more fully we understand what it is that thus remains unchanged age after age. We speak of the body as if it were a single, simple thing, to be used as a tool and then laid aside; but its complicated structure is a little world in itself. As a machine, it is such a model of compactness and ingenuity that no human skill can approach its perfection. It possesses a twofold life--a life for itself as well as a life for our use. In its own proper life it carries on a thousand curious operations necessary for its growth and maintenance, quite independent of our volition or consciousness. It contains extensive manufactories full of complicated and delicate machinery for the production of sugar, milk, acids, alkalies, salts; it has storehouses of iron, lime, and other chemical substances; there are magazines where it lays up supplies against a time of scarcity; it has its refiners and scavengers; apparatus for warming and ventilating; it has pumps and propellers constantly at work, and a more perfect electrical apparatus than has ever been invented. All these remarkable operations are directed by intelligence, working according to a plan, and combining these manifold energies for one purpose--viz., the maintenance, during a certain period, of a healthy human body. Besides this independent existence of its own, the body possesses a life of relation, by means of which it is fitted to the uses of individual and social existence. Its powers of locomotion, its active senses, its faculty of feeling, its wonderful human hand, and its still more wonderful human brain, all belong to this other use of the body as an instrument for the expression of intelligence and emotion.