View of the intercostal muscles which fill up the interspaces between the ribs. These muscles consist of a double layer of fibres, the external and the internal, which cross or intersect each other.
382. One part of the action of inspiration consists, then, of this ascent of the ribs. The ascent of the ribs is effected by the contraction of a double layer of muscles called the intercostal (fig. [CXLII]. ), placed in succession between each rib; and which communicate this motion in the following mode. The first rib is fixed; the second rib is moveable, but less moveable than the third, the third than the fourth, and so on through the series: consequently the contraction of the intercostal muscles (figs. [CXLII]. and [CXLVI]. 2) must elevate the whole series, because the upper ribs afford fixed points for the action of the muscles; and so, when all these muscles contract together, they necessarily pull the more moveable arches upwards towards the more fixed (figs. [CXLI]. and [CXLVI]. 2).
383. But from the oblique direction of the ribs, they cannot ascend without at the same time protruding forwards their anterior extremities (fig. [CXLI]. ). Those extremities being attached to the sternum, which forms the anterior wall of the thorax, they cannot be protruded forwards without at the same time carrying the sternum forwards with them (fig. [CXLI].). Thus, by this two-fold motion of the ribs, an upward and consequently an outward motion, the capacity of the thorax is increased from behind forwards, that is, in its small diameter.
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.
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.