We have seen that respiration is divided into two movements—inspiration and expiration. In inspiration, the diaphragm contracts and sinks down, pushing the abdominal organs downward. The ribs rise by the contraction of numerous muscles; at the same time with the sternum, which is carried forward, the intercostal spaces enlarge, and the chest is developed in all its dimensions, vertically, antero-posteriorly, and transversely. In expiration, the inspiratory muscles relax, and others, especially those of the abdomen, lower the ribs and the sternum by contracting the chest; while the lungs, distended by the air inspired, collapse under the pressure of the thoracic walls and their own proper elasticity. The experiments of Duchenne of Boulogne tend to prove that this contraction of the lungs is due to the muscular fibres which accompany the bronchia down to their minutest ramifications.
Nearly all the inspirations are effected by the movements of the diaphragm and the inferior ribs only. From time to time a deeper and more complete inspiration causes the thorax to rise, not simultaneously but successively at the base, then at the apex. In the first case the respiration is diaphragmatic; when the lower and middle ribs are raised it is termed lateral; and lastly, when the first rib and clavicle take part in the movement, it is costo-superior or clavicular. In diaphragmatic respiration, as M. Mandl has observed, the larynx is immovable, the inspiration is easy, without effort, and permits exertion in singing or in gymnastics for a long time and without fatigue. On the contrary, persons who respire principally by the upper ribs are easily fatigued, and very soon out of breath. This is seen in women when the corset compresses the base of the chest, and in singers who adopt, on erroneous principles, the bad habit of clavicular respiration. In this last method of inspiration the larynx is drawn down by the contraction of the external muscles, and its action becomes painful. The effort of the inspiratory muscles rapidly induces fatigue, and the inspiration, always incomplete, becomes also more frequent. Diaphragmatic respiration is practised by mountaineers, gymnasts, and skilful singers—a habit induced either by instinct, or a well-directed education.
The respiratory movements are not completely under the control of the will. It is not possible long to suppress the contrary movement after an inspiration, and when expiation has taken place the need of inspiration soon makes itself imperiously felt It is impossible, in fact, to retain the bream except for a very short space of time, two or three minutes at the longest: the most thoroughly trained divers not being able to exceed this limit.