But what is blood? It is a fluid. It runs about like water, but while water is transparent, blood is opaque. Under a microscope you will see a number of little round bodies—the blood-discs, or blood-corpuscles. All the redness there is in blood belongs to these. These red corpuscles are not hard, solid things, but delicate and soft, yet made to bear all the squeezing they get as they drive around the body. Besides these red corpuscles, are other little bodies, just a little larger than the red, not colored at all, and quite round. These are all that one can see in blood, but it has a strange property which we will study. Whenever blood is shed from a living body, within a short time it becomes solid. This change is called coagulation. If a dish be filled with blood, and you were to take a bunch of twigs and keep slowly stirring, you would naturally think it would soon begin to coagulate; but it does not, and if you keep on stirring you find that this never takes place. Take out your bundle of twigs, and you will find it coated all over with a thick, fleshy mass of soft substance. If you rinse this with water you will soon have left nothing but a quantity of soft, stringy material matted among the twigs. This stringy material is, in reality, made up of fine, delicate, elastic threads, and is called fibrin; by stirring you have taken it out. If the blood had been left in the dish for a few hours, or a day, you would find a firm mould of coagulated blood floating in a colorless liquid. This jelly would continue shrinking, and the fluid would remain; this fluid is called serum, and it is the blood out of which the corpuscles have been strained by the coagulation. All these various things, fibrin, serum, corpuscles, etc., make up the blood. This blood must move, and how does it move? You have had the different organs which assist in its circulation described, but let us illustrate.