The heart is a muscular pump that serves two functions: (1) to collect blood from the tissues of the body and pump it to the lungs and (2) to collect blood from the lungs and pump it to all tissues of the body. The human heart lies in the protective thorax, posterior to the sternum and costal cartilages, and rests on the superior surface of the diaphragm. The heart assumes an oblique position in the thorax, with two-thirds to the left of midline. It occupies a space between the pleural cavities called the middle mediastinum, defined as the space inside the pericardium, the covering around the heart. This serous membrane has inner and outer layers, with a lubricating fluid in between. The fluid allows the inner visceral pericardium to glide against the outer parietal pericardium.