The heart has a covering as well as a lining, the pericardium, consists of two layers of fibrous tissue with a small space in between. The inner layer covers the heart like a layer of skin, but the outer layer is much looser, allowing room for the heart to beat. A thin layer of pericardial film fills the space between the two layers and acts as a lubricant between the heart and its enveloping sac. It also supports the heart and cushions it as it beats.
