The muscular movement that enables the passage of air to and from the lungs or other respiratory organs of an animal. The mechanism of the movement varies with the species. In insects abdominal muscles relax and contract rhythmically to encourage the flow of air through the tracheae. In amphibians air is drawn into the lungs by a pumping action of the muscles in the floor of the mouth. Breathing in mammals involves the muscle of the diaphragm and the intercostal muscles between the ribs. Contraction of these muscles lowers the diaphragm and raises the ribs, so that the lungs expand and air is drawn in (see inspiration). Relaxation has the opposite effect and forces air out during expiration. See also ventilation centre.