According to the Encyclopaedia Britannica, the human heart is located slightly to the left of the center of the two lungs and behind the breastbone. The heart rests on the diaphragm, which is the muscular partition between the chest and the abdominal cavity.
The Encyclopaedia Britannica states that the heart consists of several layers of a tough muscular wall, which is called the myocardium. A thin layer of tissue called the pericardium covers the outside. The endocardium is the thin layer of tissue that lines the inside. The heart cavity is divided in the middle into a left and a right heart. The left and right heart is subdivided into two chambers. The upper chamber is called an atrium or auricle, and the lower chamber is called a ventricle.