The frequently branching arterioles finally terminate in the capillaries, in which distinct branches can no longer be recog nized, but the thin canals are interwoven into a network of blood channels, the meshes of which are made up of vessels, all of which have about the same calibre. They communicate indefinitely with the capillary meshworks of the neighboring arterioles, so that any given capillary area appears to be one continuous network of tubules, connected here and there with the similar networks from distinct arterioles, and thus any given capillary area may be fed with blood from several different sources. The walls of the capillaries are composed of a single layer of elongated endothelial cells (possibly lining an invisible membrane) cemented edge to edge to form a tube. They are soft and elastic, and permeable not only to the fluid portion of the blood, but also, under certain circumstances, to the corpuscles.
Fig. 127. Capillary Network of Fat Tissue. (Klein).
It is, in fact, in these networks that the essential function of the circulation is carried on, viz., the establishment of a free interchange between the tissues and the blood.
The characters of the capillary network vary in the different tissues and organs; the closeness and wideness of the meshes may be said to be in proportion to the functional activity or inactivity of the organ or tissue in question, a greater amount of blood being required in the parts where energetic duties are performed.