Vessels within the walls of larger vessels are the vasa:

1. Vessel walls are living tissue needing supply. 2. The name means vessels of the vessel. 3. Larger arteries and veins contain small blood vessels in their walls known as the vasa vasorum. 4. So the answer is vasorum. _Source: OpenStax 'Anatomy and Physiology 2e', Ch 20 'The Cardiovascular System: Blood Vessels and Circulation'_