The fluid which circulates in the lymphatic vessels of the body. It is colourless and contains salts similar to those of blood plasma, and the same proteins, although in smaller amounts, as well as LYMPHOCYTES (white blood cells). After meals, in some lymphatic vessels, the lymph contains a large amount of FAT in the form of a fine milky emulsion. These are the vessels which absorb fat from the food passing down the INTESTINE, and convey it to the thoracic duct; they are called lacteals because of the milky appearance of their contents (see CHYLE).

The lymph is derived, initially, from the blood, the watery constituents of which exude through the walls of the CAPILLARIES into the tissues, conveying material for the nourishment of the tissues and absorbing waste products.

Lymph circulates partly by reason of the pressure at which it is driven through the walls of the blood capillaries, but mainly as a consequence of incidental forces. The lymph capillaries and vessels are copiously provided with valves, which prevent any back flow of lymph, and every time these vessels are squeezed (as by the contraction of a muscle, or movement of a limb) the lymph is pumped along.

The term lymph is also applied to the serous fluid contained in the vesicles which develop as the result of VACCINATION, and used for the purpose of vaccinating other individuals.