From the case:
Transverse pericardial sinus (Gray's illustration)
{"current_user":null,"step_through_annotations":true,"access":{"can_edit":false,"can_download":true,"can_toggle_annotations":true,"can_feature":false,"can_pin":false},"extraPropsURL":"/studies/74761/annotated_viewer_json?lang=us"}
Posterior wall of the pericardial sac, showing the lines of reflection of the serous pericardium on the great vessels. (Transverse sinus labeled at center. Oblique sinus not labeled, but visible inferior to transverse sinus between the right and left pulmonary veins).
Case Discussion
This image is from Wikipedia and Gray’s Anatomy (public domain, out of copyright).