The Veins
Although thinner walled, the larger veins have a construction similar to that of arteries. The smallest ones, the venules, may pass through several successive confluences before acquiring smooth muscle.
The tunica interna of veins is always thin, lacks an elastic membrane, and is involved in the formation of the valves (see p. 25). 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 the specific significance of structural adaptations is not well understood. However, clear indications exist that the muscular layer can increase in thickness in response to elevated venous pressure (e.g., the digital veins of horses).
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