The work is done by the arterial blood brought to it by a great branch direct from the aorta, known as the hepatic artery, minute branches of which in the form of capillaries, spread themselves around the hepatic lobules.

They begin in the minute capillaries at the extremities of the four limbs, and everywhere throughout the body, and passing onwards toward the heart, receive constantly fresh accessions on the way from myriad other veins bringing blood from other wayside capillaries, till the central veins gradually unite into larger and larger vessels until at length they form the two great vessels which open into the right auricle of the heart.

74.The Structure of Capillaries.

Capillaries of various sizes, showing cells with nuclei] 189.

The Capillaries.

The capillaries are the minute, hair-like tubes, with very thin walls, which form the connection between the ending of the finest arteries and the beginning of the smallest veins.

In fact, the capillaries form a network of the tiniest blood-vessels, so minute as to be quite invisible, at least one-fourth smaller than the finest line visible to the naked eye.

The capillaries serve as a medium to transmit the blood from the arteries to the veins; and it is through them that the blood brings nourishment to the surrounding tissues.

The walls of the capillaries are of the most delicate structure, consisting of a single layer of cells loosely connected.

The number of the capillaries is inconceivable.

The thin walls of the capillaries are admirably adapted for the important interchanges that take place between the blood and the tissues.

[Illustration: Fig. 79.Circulation in the Capillaries, as seen with the Microscope.]

Now, as we have seen, the change from venous to arterial blood occurs in the capillaries of the lungs, the only means of communication between the pulmonary arteries and the pulmonary veins.

The use of intoxicants involves a repeated dilatation of the capillaries, which steadily diminishes their defensive power, rendering the person more liable to yield to the invasion of pulmonary diseases. 230.

From this tissue activity, which is mainly oxidation, are formed certain waste products which, as we have seen, are absorbed by the capillaries and lymphatics and carried into the venous circulation.

The thin wall of the capillaries within each capsule separates the blood from the cavity of the tubule.

But the delicate epithelial walls of the tubules through which it passes permit the inflow of urea and other waste products from the surrounding capillaries.

The blood which has escaped from the capillaries is slowly absorbed, changing color in the process, from blue black to green, and fading into a light yellow.

If the capillaries are injured the blood merely oozes.

Hemorrhage from a vein or from the capillaries is rarely troublesome, and is ordinarily easily checked, aided, if need be, by hot water, deep pressure, the application of some form of iron styptic, or even powdered alum.

The essential parts of a gland consist of a basement membrane, on one side of which are found actively growing cells, on the other is the blood current, flowing in exceedingly thin-walled vessels known as the capillaries.

THE ANATOMY AND PHYSIOLOGY OF CAPILLARIES, by August Krogh.

SEE The anatomy and physiology of capillaries.

The anatony and physiology of the capillaries.

The anatomy and physiology of the capillaries.