Tunica adventitia: entirely made of connective tissue. It also contains nerves that supply the muscular layer, as well as nutrient capillaries (vasa vasorum) in the larger blood vessels.

• 外膜：完全由结缔组织构成。并含有支配肌层的神经。在较大血管中含有营养毛细血管（血管滋养管）。