Last Tango in Paris

Marcus can do no wrong. As a school boy he won all the awards, then matriculated to a top college, and went on to graduate suma cum laude from his medical school. Of course, he chose to become a thoracic surgeon. His one flaw was he could not tango. Try as he might, whenever he performed a pasada, his milongesa stood a good chance of landing hard on the dance floor. Indeed, he had to seek out clubs in various cities because knowledge of his ineptitude preceded him, and he was barred from every milonga along the East coast. As it so happens, during one of these mishaps, his partner collapsed mid cruce forzado and began to demonstrate symptoms of a cardiac event. Marcus sprang into action and took his compromised partner to the hospital where he diagnosed an atrial septal defect (using only his stethoscope, of course). Marcus requested an OR and prepared to fix the defect. He cracked her chest, but spotted a persistent left superior vena cava draining to the coronary sinus. Marcus knew he must repair the defect carefully so as not to injure the coronary sinus that is receiving the venous return of the persistent left superior vena cava. Where on the heart did Marcus demonstrate extreme care to avoid injuring the coronary sinus?

Postscript: Patient survived, woke up to an elated Marcus, but refused to accept his invitation for a Last Tango in Paris. Instead, she offered to teach him how to play the castanets.

Coronary sinus is associated with which labeled line?