As the arteries proceed to the proper tissues of the body, they divide and subdivide till they are no longer visible to the unassisted eye, and they finally break up into minute vessels named capillaries, the size of which is tolerably uniform for each organ. These communicate freely with each other and form a mesh or web, the arrangement of which presents variations corresponding to the structure of the organ in which they exist, being ladder-like in muscle, tuft-like in the kidney, and basketlike round the lobules of fat and of glands.
The wall of the capillaries is formed of a delicate basement membrane with a lining of flattened cells (endothelium), or even of the cells alone. Through this thin wall the white corpuscles of the blood seem to be able to bore without leaving any rent behind them, just as a needle may be thrust through the wall of a soap-bubble and withdrawn without causing the bubble to burst. The corpuscles then become free, and can move about in the interspaces of the adjoining tissues; but whether they die there or reenter the vessels is unknown. This process, in which the white corpuscles pass out of the vessels, is named diapedesis. The current of blood is not always the same in the same capillary vessel, the direction being dependent upon the increase of pressure behind, that is, on the arterial side, or the relief of pressure in front or on the venous side. In examining the circulation in the capillaries it will be seen that they are not large enough in many parts to admit two coloured corpuscles abreast, and they consequently follow each other in single file; but in the somewhat larger vessels the red corpuscles occupy the centre of the stream, whilst the white corpuscles roll lazily along at the margin in close contact with the inner surface of the wall of the vessel. It is in the play of the fluids within and without the capillary vessels that an important part of the processes of nutrition is transacted, the blood surrendering to the fluid which moistens their external wall the soluble materials for the nutrition of the tissues, whilst it takes up from that fluid the soluble products of the degeneration and decay of the tissues. The capillaries of the lungs are very large, and form a close net-work. Those of the brain are minute and less close. The average diameter of the capillaries may be taken to be a little larger than the diameter of the corpuscles of the blood of the same animal. The pressure of the blood in the capillaries is considerable, being capable of supporting a column of mercury about 1 inch in height. The blood flows through them at the rate of about one twenty-fifth of an inch in a second.
The veins have thinner walls than the arteries, are more numerous than they, and have much greater capacity, containing, according to the estimate of Haller, about twice as much blood. They communicate with each other by large branches very frequently in their passage to the heart, and in structure very much resemble the arteries. The principal instance where veins do not convey the blood directly to the heart is the great portal vein, which conducts the blood from the intestines to the liver, and there breaks up into a second system of capillaries, which unite together again to form the hepatic vein; a similar arrangement occurs in the case of the kidney.
In the veins of the neck and limbs, however, differences are found in the form of numerous valves, usually arranged in pairs, and at tolerably regular distances from each other. These valves are composed of a reflexion of the inner coat, strengthened with some connective-tissue fibres, and near their base have also a small amount of involuntary muscle. The function discharged by the veins is to convey the blood back to the heart. The veins collectively, though there are a few exceptions, convey the blood to the heart, and run side by side with the arteries after which they are named. The radial artery is thus accompanied by the radial vein or veins, the brachial artery by the brachial veins. There are often two or more veins to one artery. The veins are more exposed than the arteries, as is seen in those of the neck, face, body, and limbs. Their capacity is greater than that of the arteries, and the blood flows through them with a uniform and continuous current, but more slowly, the velocity of the current being about 1 foot per second. They communicate freely with one another, and hence obstruction in any one vein is of less importance than in the case of an artery, since the flow of blood hindered or arrested in one channel of the former easily finds escape by another.