1. References

The interatrial sulcus, separating the two atria, is scarcely marked on the posterior surface, while anteriorly it is hidden by the pulmonary artery and aorta.

References

• The Underwater Album
• The underworld
• The Undesired Princess
• The Undeveloped Heart EP
• The Undisclosed
• The Undiscovered Numbers And Colors
• The Undiscovered Numbers & Colors
• The Undiscovered Underground
• The Undisputed Truth (Brother Ali album)
• The Undretaker
• The Undying Darkness
• The Undying Monster
• The Unearthly
• The Unearthly Child
• The Uneventful Vacation
• The Unexpected
• The Unexpected (album)
• The Unexpected Man
• The Unexpected Mrs. Pollifax
• The Unexpected (novel)
• The Unexplained
• The Unexplained (magazine)
• The Unexplained (TV series)
• The Unfair Commercial Practices Directive
• The Unfairground