384. Such is the part of the action, in inspiration, performed by the motion of the ribs. The remaining part of that action, by far the most important, consists of the enlargement of the capacity of the thorax from above downwards, or in its long diameter. This is effected by the descent of the diaphragm (fig. CXLIII.). 385. The diaphragm is a circular muscle, forming a complete but moveable partition between the thorax and the abdomen (figs. CXXXIV. 10, and CXLIII. 3). When not in action its upper surface forms an arch (figs. CXLIII. 4, and CXLV. 1), the convexity of which is towards the thorax (figs. CXLIII. 4, and CXLV. 1), and reaches as high as the fourth rib (fig. CXLV. 1); its under surface, or that towards the abdomen, is concave (figs. CXXXIV. 10, and CXLV. 1). Its central portion is tendinous (fig. CXLIII. 4). This central tendinous portion of the diaphragm, which is in apposition with the heart (fig. CXXXIV. 8), and firmly attached to the pericardium (fig. CXXXIV. 7), is nearly if not quite immoveable: it is only the lateral or muscular portions (fig. CXLIII. 4) that are capable of motion. Its central portion is constructed of dense and firm tendon, and is immoveable, primarily, in order to afford one of the two fixed points (the ribs affording the other fixed point), essential to the action of the muscular fibres that constitute its lateral or moveable portions; and secondarily, in order to afford a support to the heart, which rests upon this central tendon. Thus, in consequence of this tendon being rendered absolutely fixed, the motions of the diaphragm are completely prevented from incommoding the motions of the heart; the function of respiration from interfering with the function of the circulation. [Illustration: Fig. CXLIII.—_View of the Diaphragm._ 1. Cavities of the thorax. 2. Portion of cavity of the abdomen. 3. Lateral or muscular and moveable portions of the diaphragm. 4. Central or tendinous and fixed portion of the diaphragm.] 386. During the action of inspiration the muscular or lateral portions of the diaphragm contract (fig. CXLIII. 3); its muscular fibres shorten themselves, and are approximated towards the central tendon (fig. CXLIII. 2); the consequence is that the whole muscle descends (fig. CXLIV. 1); passes from the fourth to below the seventh rib (fig. CXLIV.), loses its arched form and presents the appearance of an oblique plane (fig. CXLIV.). At the same time the muscles of the abdomen are protruded forwards (fig. CXLIV. 2), and the viscera contained in its cavity are pushed downwards. The result of these movements is, that the capacity of the thorax is enlarged by all the space that intervenes between the fourth rib (fig. CXLV. 1), and the lowest point of the oblique plane formed by the diaphragm (fig. CXLIV. 1), together with all that gained by the protrusion of the walls of the abdomen and the descent of its viscera (fig. CXLIV. 2).