=Bloodvessels.=—The greater circulation occurs through a closed system of vessels which unite the left ventricle with the right auricle. The aorta gives off lateral branches. Its branches branch. Subdivision continues until the vessels are just wide enough to allow blood-corpuscles to pass in single file, or but little wider. When a bough of a tree divides, the united cross-sections of its twigs, their soft bark being stripped off, may be a little larger than the cross-section of the bough; but the disparity is usually small. The united cross-sections of the smaller arteries is considerably greater than that of the trunks which give origin to them. By the time the capillaries are reached, their total bed—their united cross-section—is about 640 times as great as that of the aorta. This estimate is based upon the diminution in the rate at which blood flows through the vessels. The velocity with which a stream flows through a channel varies as the cross-section of the channel. In a capillary vessel the blood flows at the rate of from 0·5 millimetre to 1 millimetre per second. In the aorta the velocity is about 320 millimetres per second. In the re-formation of the venous system a converse process of reduction occurs, but not with anything like the same rapidity. The united calibre of the two venæ cavæ, in which the reduction is complete, is about twice that of the aorta. From this it follows that the veins hold much more blood than the arteries; and since veins are more easily distended, the amount that they can hold varies within wide limits. They constitute to some extent a reservoir for blood. The capillary vessels are the tubes of the circulatory system in which blood comes into use. On the average they are about 0·5 millimetre long. Through them the blood flows slowly. Through their walls alone is there any exchange worth mentioning between the blood within the vascular system and the lymph by which it is surrounded. Interest therefore centres in these vessels. Their walls are formed of endothelial tiles. In the centre of each thin transparent tile is a boss, where its lens-shaped nucleus is situate. The outline of the tile is sinuous. Its margin dovetails with the margins of those adjacent to it. Oxygen and carbonic acid, nutrient substances and waste products, pass rapidly through the endothelial cells. Leucocytes have the power of pushing the cells aside, in order that they may make their way out of the blood into the lymph which fills the tissue-spaces. With the exception of the lens and cornea of the eye, cartilage, and the various epidermal structures, all tissues are traversed by capillary vessels. It is not difficult to calculate the number of such vessels in the body exclusive of the liver and the lungs. The diameter of the aorta is 28 millimetres, that of a capillary about 0·008 millimetre. The cross-section of all the capillaries added together is 640 times that of the aorta, as already stated.