The minute vessels which join the ends of the arteries to venules, the tiny commencement of veins. Their walls consist of a single layer of fine, flat, transparent cells, bound together at the edges, and the vessels form a meshwork all through the tissues of the body, bathing the latter in blood with only the thin capillary wall interposed, through which gases and fluids readily pass. These vessels are less than 0·025 mm in width.