 Thoracic diaphragm, the thoracic diaphragm, or simply the diaphragm, is a sheet of internal skeletal muscle in humans and other mammals that extends across the bottom of the thoracic cavity. The diaphragm separates the thoracic cavity, containing the heart and lungs, from the abdominal cavity and performs an important function in respiration. As the diaphragm contracts, the volume of the thoracic cavity increases and air is drawn into the lungs. The term diaphragm in anatomy can refer to other flat structures such as the urogenital diaphragm or pelvic diaphragm, but the diaphragm generally refers to the thoracic diaphragm. In humans, the diaphragm is slightly asymmetric – its right half is higher up superior to the left half, since the large liver rests beneath the right half of the diaphragm. There is also a theory that the diaphragm is lower on the other side due to the presence of the heart. Other mammals have diaphragms, and others vertebrates such as amphidians and reptiles have diaphragm-like structures, but important details of the anatomy vary, such as the position of the lungs in the abdominal cavity.