Light micrograph of a longitudinal section of a large vein showing, from the top, the tunica intima with the endothelium and a thin subendothelial connective tissue. There is a middle layer formed by a few bundles of circular cross-sectioned myocytes. The majority of the wall is occupied by intermingled bundles of collagen fibres belonging to the tunica adventitia. A small blood vessel (the vasa vasorum system) can be seen in the adventitia.