The respiratory tract of elephants is comprised of the conducting portion (external nares, nasal tubes, internal nares, pharynx, larynx and trachea) and the respiratory portion (bronchi, bronchioles, alveolar duct and alveolar sac in lung). Sound is produced from larynx. The lungs are attached to the thoracic walls and diaphragm, oblitherating the potential pleural space normally maintained in negative pressure to assist breathing in other mammals. . Unlike most other mammals, elephants rely on intercostals and diaphragmatic muscle movement alone to inflate and deflate the lungs. If there is an impediment in the muscular excersion of the key muscles in respiration for the elephant, the resulting dyspnea is severe. For example, long periods of sternal recumbency increase abdominal pressure thereby limiting diaphragm motion. Elephants poorly tolerate sternal recumbency and assume lateral recumbency when they lay down. This can be observed at times when elephants sleep.