The ribs are united by intercostal muscles, disposed in two sheets. The fibres of the external intercostals are directed downwards and forwards, those of the internal intercostals downwards and backwards. In tranquil respiration the chest is enlarged by the external intercostal muscles, which raise the ribs, and the diaphragmatic muscle, which renders peripheral portions of the diaphragm flat. The rôle of the internal intercostal muscles is a subject still under discussion. For the most part, physiologists regard them as accessory to expiration, but some hold that they combine with the external intercostals in raising the ribs and twisting them outwards during inspiration. The diaphragm is a partition which separates the thoracic from the abdominal cavity. It is in the form of a vault. The central portion of the dome is membranous, its margins muscular. Its membranous centre is in contact with the pericardium, which encloses the heart. The level of this part is therefore fixed, except in forced inspiration, when it descends slightly. It constitutes a fixed plane for the muscles of the diaphragm, which are attached below to the vertebral column and the ribs. When the muscles contract in inspiration, the curvature of the marginal portions of the diaphragm is diminished, and the chest-cavity consequently enlarged. During expiration the space between the muscle of the diaphragm and the chest-wall closes up, and the lower border of the lung slips out of it. There is a marked difference in the relative extent of the costal and diaphragmatic movements in men and women. In women respiration is chiefly costal; in men it is chiefly diaphragmatic. In men the abdomen moves forwards, as the diaphragm descends in tranquil breathing; in women the chest rises. Men who wish, for the purposes of athletics, or singing, or public speaking, to retain the power of making the most of their chest-capacity are wise in not allowing themselves to fall into the habit of lazy, abdominal breathing. [Illustration: FIG. 8.—THE DIAPHRAGM AND ORGANS IN CONTACT WITH IT—A, IN EXPIRATION; B, AT THE END OF A DEEP INSPIRATION. TRANSVERSE VERTICAL SECTIONS IN THE LINE OF THE ARMPIT. A, At the end of an ordinary expiration the lung does not extend below the upper border of the eighth rib. From this level to the middle or lower border of the tenth rib the two layers of the pleura covering respectively the inner wall of the chest and the upper surface of the diaphragm are in contact. B, When the lung is distended with air it occupies the whole of the pleural cavity.] When additional pressure is required, when respiration is forced, various external muscles attached to the spinal column, the shoulder-blades, and the clavicles, as well as the muscles of the abdomen, come into play.