375. These organs, which invariably act in concert, are so constructed and disposed, that when in action they give to the chest two alternate motions, one that by which its capacity is enlarged; and the other that by which it is diminished. These alternate movements are called the motions of respiration. The motion by which the capacity of the chest is enlarged is termed the action of inspiration, and that by which it is diminished the action of expiration. 376. The action of inspiration, or that by which the capacity of the chest is enlarged, is effected by the combined movements of the thorax and diaphragm; by the ascent of the thorax and by the descent of the diaphragm. 377. The osseous portion of the thorax, which has been fully described (69 _et seq._), consists of the spinal column (fig. CXLI. 1), the ribs with their cartilages (fig. CXLI. 2, 3), and the sternum (fig. CXLI. 4). The soft portion of the thorax consists of muscles and membrane (figs. CXLII., CXLVI., and CXLVII.), together with the common integuments of the body. The chief boundaries of the cavity of the thorax before, behind, and at the sides, are osseous, being formed before by the sternum and the cartilages of the ribs (fig. CXLI. 4, 3); behind by the spinal column and the necks of the ribs (fig. CXLI. 1,2); and at the sides by the bodies of the ribs. Below the boundary is muscular, being formed by the diaphragm (fig. CXLIII. 3). 378. Externally the thorax is convex and enveloped by muscle and skin; internally it is concave (fig. CXLIII. 1), and lined by a continuation of the same membrane which envelops the lungs, the pleura (104). But that portion of the pleura which lines the internal wall of the thorax is called the costal pleura (pleura costalis), in contradistinction to that which envelops the lungs, which is termed the pulmonary pleura, or pleura pulmonalis (104). By the costal pleura, a thin but firm and strong membrane, smooth, polished, and like all the membranes of its class (serous membrane 30, _et seq._), kept in a state of perpetual moisture and suppleness, by a fluid secreted at its surface, the movements of the thorax are facilitated, at the same time that they are prevented from injuring the delicate organs contained in it. 379. The moveable parts of the osseous portion of the thorax are the ribs and sternum. The ribs, though by one extremity tied with exceeding firmness to the spinal column by ligaments specially constructed, and admirably adapted for that purpose (figs. LVI. 1, and LVII. 1), and though attached at their other extremity by their cartilages to the sternum (fig. LVIII.), are capable of three motions, an upward, an outward, and a downward motion. [Illustration: Fig. CXLI.—_View of the osseous portion of the Thorax._ 1. Spinal column. 2. Ribs. 3. Cartilages of ribs. 4. Sternum.]