Capillary Vessels (Lat. capillaris, hairlike), the small vessels intermediate between the arteries and the veins; so called from their minute size, and from the fact that they are all of nearly unvarying diameter, as compared with the larger vessels. The capillary blood vessels are composed of a delicate, transparent, elastic tubular membrane, marked at various points with small oval spots termed nuclei. Their average diameter in the human subject is about 1/3000 of an inch. They inosculate with each other very abundantly, making numerous communications in every direction at very short intervals, and form in this way what is known as the capillary plexus or network. In some organs, as in the lungs, these communications are so abundant that the interspaces between the vessels are hardly so extensive, when taken together, as the capillaries themselves. This frequent intercommunication and inosculation of the capillary vessels is their most important characteristic as distinguished from the arteries and the veins.

For while the arteries constantly divide and separate from each other so as to convey the blood to separate organs, and the veins unite into larger branches and trunks in order to collect the blood from the different organs and return it to the heart, the capillaries, on the other hand, are so arranged as to disseminate the blood in a multitude of minute streams through the substance of an organ, and thus to bring it into intimate contact with its tissue. This form of the capillary plexus varies in different parts. In the cellular tissue the meshes are irregular in shape; in the muscles they are oblong, in the papillae of the skin and tongue they are arranged in loops, in the Mal-pighian bodies of the kidney they form convoluted globular tufts, and in the glandular organs they surround the secreting follicles with a vascular network. In all cases the capillary vessels receive their blood from the arteries and deliver it into the veins. The only apparent exception to this is in the capillary plexus of the liver, which is supplied with blood in great part from the portal vein, which has already collected it from the capillary system of the stomach and intestines.

The liver, however, is also supplied with blood by the hepatic artery, and even the blood of the portal vein, though immediately derived from the capillaries of the alimentary canal, was yet first transmitted from the arteries of these organs.