The diaphragm is the primary muscle used in the process of inspiration, or inhalation. It is a dome-shaped sheet of muscle that is inserted into the lower ribs. Lying at the base of the thorax (chest), it separates the abdominal cavity from the thoracic cavity. It is a thin, skeletal muscle that can contract voluntarily. Located beneath it are the abdominal muscles. Serving as the inferior aspect of the thorax, it is the means by which the chest cavity volume is increased. In order to expand the thorax, increase the vertical dimension of the chest, and decrease the air pressure in the thoracic cavity, this sheet of muscle has to flatten out by contracting. When the air pressure falls, air will rush into the respiratory tract. During contraction, the diaphragm pushes downward and pulls the pleura, a serous membrane surrounding the lungs, with it. This action causes the pleural pressure and the alveolar pressure to drop, which in turn, facilitates air to flow into the lungs.