THE VEINS
Although thinner walled, the larger veins have a construction similar to that of arteries. The smallest ones, the venules, do not possess muscle and may pass through several successive confluences before acquiring this component of the wall.
The tunica interna is always thin and lacks an elastic membrane; its chief distinction is its involvement in the formation of the valves whose form, disposition, and function have already been noted (p. 27). The media is relatively weak, is mainly muscular, and has little admixture of elastic elements. Elastic fibers are more plentiful in the adventitia.The structure of veins is much less uniform than that of arteries, but although many specializations have been described, it has not yet been possible to assign specific adaptive significance to all. However, longitudinal bundles of smooth muscle within the adventitia of some veins can be correlated with a capacity to alter in length with changes in circumstance. Clear indications exist that the muscular layer can increase in thickness in response to elevated venous pressure (e.g., the digital veins of horses).