A 65 year old female, with a past medical history of alcohol…

A 65 year old female, with a past medical history of alcoholism and hypothyroidism, presents to her primary care doctor with a complaint of shortness of breath.  Her chest x-ray demonstrates a large cardiac silhouette, suggesting dilated cardiomyopathy.  What comprises the largest portion of the sternocostal surface of the heart seen on x-ray?