"The blood is a thick, opaque fluid, varying in color in different parts of the body from a bright scarlet to a dark purple, or even almost black." If a drop of blood be placed under a microscope, immense numbers of small bodies will be seen. These are called blood-globules, or corpuscles, or discs. There are both red, and white or colorless, corpuscles. Each red corpuscle is soft and jelly-like. Its chief constituent, besides water, is a substance called hemoglobin, which has the power of combining with oxygen when in a place where that gas is plentiful, and of giving it off again in a region where oxygen is absent, or present only in small quantity. Hence, as the blood flows through the lungs, which are constantly supplied with fresh air, its corpuscles take up oxygen, which, as it flows on, is carried by them to distant parts of the body where oxygen is deficient, and there given up to the tissues. This oxygen-carrying is the function of the red corpuscles. Hemoglobin, as the coloring-matter of the blood is called, is dark purplish-red in color; combined with oxygen it is bright "scarlet red." Accordingly, the blood which flows to the lungs after giving up its oxygen is dark red in color, its dark color being due to the impurities it contains; and that which, having received a fresh supply of oxygen, flows away from the lungs is bright scarlet--having been cleansed of its impurities. The bright red blood is called _arterial_, and the dark red _venous_. The work assigned to the blood in the economy of the human system is: first, to pick up nutriment in its course through the walls of the alimentary canal, and oxygen, as it flows through the lungs, and convey these to all other parts of the body. Second, to act as a sort of sewage stream that drains off waste matter, and to carry this to the organs of excretion by which waste is expelled from the body. "The blood is the great circulating market of the body, in which all the things that are wanted by all parts, by the muscles, the brain, the skin, the lungs, liver and kidneys, are bought and sold. What the muscles want they buy from the blood; what they have done with, they sell back to the blood; and so with every other organ and part. As long as life lasts this buying and selling is forever going on, and this is why the blood is forever on the move, sweeping restlessly from place to place, bringing to each part the thing it wants, and carrying away those with which it has done. When the blood ceases to move, the market is blocked, the buying and selling cease, and all the organs die, starved for lack of the things they want, choked by the abundance of things for which they have no longer any need."--FOSTER.