Lymph is the ﬂuid which circulates in the lymphatic vessels of the body. It is a colourless ﬂuid, like blood PLASMA in composition, only rather more watery. It contains salts similar to those of blood plasma, and the same proteins, although in smaller amount: FIBRINOGEN, serum albumin (see ALBUMINS), and serum GLOBULIN. It also contains lymphocytes (white blood cells), derived from the glands. In some lymphatic vessels, the lymph contains, after meals, a great 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 their contents look milky (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.
The spaces in the tissues communicate with lymph capillaries, which have a structure similar to that of the capillaries of the blood-vessel system, being composed of delicate ﬂat cells joined edge to edge. These unite to form ﬁne vessels, resembling minute veins in structure, called lymphatics, which ramify throughout the body, passing through lymphatic glands and ultimately discharging their contents into the jugular veins in the root of the neck. Other lymph vessels commence in great numbers as minute openings on the surface of the PLEURA and PERITONEUM, and act as drains for these otherwise closed cavities. When ﬂuid is eﬀused into these cavities – as in a pleural eﬀusion, for example – its absorption takes place through the lymphatic vessels. The course of these vessels is described under the entry on GLAND.
Lymph circulates partly by reason of the pressure at which it is driven through the walls of the blood capillaries, but mainly in 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.