The colorless fluid within these vessels is termed "lymph." At intervals along these tubes are small structures termed the lymph nodes, which essentially are filters, and strain out from the fluid substances which might work great injury if they passed into the blood.

Between the capillary vessels and the lymphatics is the tissue fluid, in which all the exchange takes place.
It is constantly added to by the blood, and returns fluid to the blood and lymph; it gives material to the cells and receives material from them.
[Illustration: FIG.

11 .-- A DIAGRAMMATIC VIEW OF THE BLOOD VESSELS.

An artery (_a_) opens into a system of capillaries, (_c_) and after passing through these collects into a vein (_b_).

Notice that the capillaries connect with other vascular territories at numerous points (_d_).