The tunica adventitia of the big veins was very thick and was comprised of abundant smooth muscular bundles, collagen and elastic fibers.

• 大静脉的内膜与中膜均较薄，中膜由数层平滑肌构成，外膜则很厚，由大量平滑肌束及胶原纤维、弹性纤维构成。