base of heart

n.
  1. The part of the heart formed mainly by the left atrium and to a lesser extent by the posterior part of the right atrium, directed backward and to the right, and separated from the vertebral column by the esophagus and aorta.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.