=The Structure of the Capillaries and Arteries.=—The capillaries are minute vessels which connect the _arterioles_, or terminal arteries, with the _venules_, or terminal veins. They are only from 6 to 14 microns in diameter. Their walls consist of a single layer of endothelial cells, which are somewhat elongated in the long axis of the vessels. Their edges are serrated, and are united by a small amount of intercellular cement-like substance. Capillaries branch without diminution in caliber, and these branches anastomose to form capillary networks, the meshes of which differ in size and shape in different tissues and organs. The largest meshed networks occur in the serous membranes and in the muscles; and the smallest occur in the glands, such as the liver. The _walls of the arteries_ are thick and stand open when empty, owing to the elastic tissue contained in their walls, while the walls of the veins collapse when empty, owing to their containing a smaller amount of elastic tissue. The arterial wall is provided with three coats: _tunica intima_, or inner coat; _tunica media_, or median coat; and _tunica adventitia_, or outer coat. The _tunica intima_ consists of a single layer of endothelial cells, continuous with and similar to that forming the walls of the capillaries. In passing from the capillaries to the arterioles, there is first a thin coat, or sheath-like layer, of connective tissue around the outside of the endothelial tubes. Further along, isolated smooth muscle cells arranged in a circular manner occur between the endothelial layer and the layer of connective tissue, this structure forming vessels called precapillary arteries. Further along still, the muscle cells form a complete layer; in this section the vessels are called arterioles and are made up of three coats: the inner endothelial, the middle muscular, and the outer fibrous. In arteries of medium size the intima consists of the endothelial layer, a layer of delicate white and elastic fibers, connective-tissue cells, and the membrana elastica interna, or an outer layer, the elastic layer, of the intima. The _media_ consists of a thick coat of circularly arranged smooth muscle cells, its thickness depending largely upon the size of the vessels. There is also a small amount of fibrillary connective tissue, which supports the muscle cells. Elastic tissue is present in the media, the amount depending on the size of the vessel, the larger the vessel the more elastic tissue there is present. In the large arteries coarse elastic fibers intermingle with the finer ones. When much elastic tissue is present the muscle cells are separated into more or less well-defined groups.