=How the Blood Vessels are Controlled.=--Evidently the blood vessels are not regulated by the will. We cannot voluntarily increase the beating of the heart, or cause it to slacken its action. Even an actor cannot cause his face to turn pale or to blush at will. This is because the tiny muscles in the walls of the blood vessels are involuntary muscles. They are controlled by nerves of the sympathetic system called vaso-motors. They are not subject to the will (see Fig. 25). The nerve center which controls the blood vessels is located in the top of the spinal cord at the base of the brain. When cold air strikes the skin the nerves near the arteries are stimulated, the arteries in the skin contract, and the skin turns white. When the heat from a hot fire strikes the skin, the nerves are soothed, the arteries relax, and the face becomes red. When the stomach is filled with food, the heart beats faster and sends more blood to aid in digestion. When we run fast, the heart beats fast to supply more blood to the muscles, but it slows down as sleep comes on, that the body and brain may rest. =Parts of the Blood.=--The blood which flows from a cut finger seems to be a bright red throughout. When a drop of it is looked at through a microscope, however, the liquid itself is seen to be almost as clear as water. This liquid is called the _plasma_. Floating in it are millions of biconcave disks containing a pigment (hemoglobin) which gives the red color to the blood. The disks are called _red corpuscles_ (Fig. 60). A few irregularly shaped bodies, nucleated and almost transparent, and called _white corpuscles_, are also found in the blood. The red corpuscles go only where the plasma carries them (Exps. 3, 4). The white corpuscles sometimes leave the blood vessels entirely. At times one may be seen shaped like a dumb-bell, half of it through the wall of the blood vessel and half still in the blood vessel. After the corpuscle is out, no hole can be found to account for its mysterious passage. _The white corpuscles consist of protoplasm. The red corpuscles contain no protoplasm. Hence the latter are not really alive._ [Illustration: FIG. 60.--HUMAN BLOOD CELLS (magnified 40,000 areas), showing many red cells and a single white blood cell on left, larger than red cells. (Peabody.)] [Illustration: FIG. 61.--SIDE AND FRONT VIEWS OF FROG’S AND MAN’S RED CORPUSCLES, drawn to same scale. Compare outline, concavity, diameters.]