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 passing through the capillaries of the lungs, the blood goes through the pulmonary veins to the left auricle (Fig. 54), then through the bicuspid or mitral valve, to the left ventricle, whence it is forced through a semilunar valve into the largest artery of the body, called the great aorta (Fig. 54). Thence it goes to the smaller arteries, and then to the capillaries of the tissues in general, thus completing the circuit.

(plan), showing the left ventricle at the moment when relaxing and receiving the blood from the auricle; and the same at the beginning of contraction to send blood into aorta. Notice action of the valve.

Structure of Veins and Arteries.—Seen under the microscope the arteries and veins show that they are made of three kinds of tissues arranged in three coats (Fig. 56): a tissue resembling epithelial tissue (Chap. 1), as a lining to lessen friction; an outer connective tissue (Chap. 1), to give elasticity; and a middle coat of muscular tissue to enable the vessels to change in size. Let us see why blood vessels must have these three properties?

Why the Blood Vessels must be Elastic.—The aorta and its branches are always full of blood. When the left ventricle with its strong, muscular walls contracts, the blood in the aorta and small blood tubes cannot move forward fast enough to make room for the new supply so suddenly sent out of the ventricle. Where can this blood go? If a