LYMPH, ITS FORMATION AND MOVEMENT. In order to gain an insight into the significance of lymph it is neces sary to consider the whole question of fluid circulation in the tis sues. The most convenient point at which to commence such a review is the blood. The function of this medium is to transport oxygen and nutritive materials to tissues requiring them and to remove certain waste products such as carbon dioxide. Blood is conveyed in a totally closed circulatory system, the nearest point of approach to the actual tissue cells being in the capillary area. The materials conveyed by the blood have to be transferred from the interior of this capillary system through a wall of thin endo thelium to fluid surrounding the cells and in intimate contact with them. This fluid, which is known as tissue fluid, acts as a middleman, conveying substances from the fluid of the blood directly to the cells of the organ.
With the important exceptions of the central nervous system the skeletal muscle bundles and spleen pulp, all tissues and organs are supplied with another set of capillary vessels in intimate con tact with the tissue fluid, but, as in the case of the capillary blood vessels, separated from it by a continuous wall of thin endo thelium. These vessels are the lymphatic capillaries. They begin as blind extremities and unite, usually with a very extensive anastomosis, to form plexuses of larger vessels with thicker walls. These in turn are drained by the lymphatic trunks which usually course with the main blood vessels of the part. The main trunks coming from the hind limbs and abdominal organs are united into one large vessel, the receptaculum chyli from which the thoracic duct passes up on the left side of the oesophagus to drain into the venous system on the left side of the neck. A smaller duct drains the right side of the head and neck and right upper limb, emptying into the veins in an analogous position on that side.
The lymph vessels coming from organs or limbs pass through lymphatic glands at some part of their course. This whole net work of vessels and glands is known as the lymphatic system. Early work on this system gave rise to the view that the lymph capillaries were in direct communication with the tissue spaces.
Lymph therefore was simply tissue fluid which had passed through openings in the walls of the smallest lymphatic vessels. Later work substituted the opinion that lymph capillaries were totally closed by endothelial plates in the same way as blood capillaries. This view, which has been restated and strengthened by the observations of Sabin, is that now generally held to be correct.
One is therefore left with the conception that the tissue of an organ consists of a collection of cells completely surrounded by a fluid medium—the tissue fluid. As a channel for the supply of nutriment there are blood capillaries. As a channel of exit only there are the lymph capillaries, and, as we shall see, the blood vessels also subserve the same function. The fluid, therefore, which is derived from the tissue fluid, after passage through the endothelial wall of the lymphatic, is lymph. Obviously its corn position relative to tissue fluid depends on the properties of the lymphatic endothelium.
It is with the composition, mode of production, function and propulsion of this fluid that we have to deal.