ENDOTHELIUM. Lining the blood vessels, the lymphatics and lymph spaces, and the pleural, pericardial, and peritoneal cavities are found flattened cells (endothelial cells) joined at their edges to form an extremely thin membrane.
An endothelial cell is very flattened and possesses a protoplasm with faint granules and an oval or round nucleus (figs. I and 2).
The cells are irregularly polyhedral and united at the edges by cement substance which, though scanty, stains with silver nitrate when the appearance reproduced in the figure is seen. By being thus united together the cells form a continuous layer. In ser ous cavities this layer is pierced by small openings (stomata), which bring the cavity into com munication with lymph spaces or vessels lying beneath the mem brane. The stomata are surrounded by a special layer of cubical and granular cells.
By means of endothelial membranes the surfaces of the parts covered by them are rendered very smooth. Thus the abdominal organs can glide easily over one another within the peritoneal cavity; the blood or lymph experiences the least amount of f ric tion, and friction is reduced to a minimum between a tendon and its sheath or in the joint cavities. The cells forming these mem branes also possess further physiological properties. Thus it is most probable that they play an active part in the blood capillaries in transmitting substances from the blood into the tissue spaces, or conversely in preventing the passage of materials from blood to tissue space or from tissue space to blood. Hence the fluid of the blood and that of the tissue space need not be of the same chemical composition. Endothelial cells also may be phagocytic (see PHAGOCYTOSIS).