380. The ribs form a series of moveable arches, the convexity of the arches being outwards, and the whole being disposed in an oblique direction (fig. CXLI. 2). The first rib springs from the vertebral column at nearly a right angle (fig. CXLI. 2); the acuteness of this angle increases in succession as the ribs descend from the first to the last (fig. CXLI. 2); in this manner each rib is inclined obliquely outwards and downwards, and the obliquity thus given to the general direction of the ribs augments progressively from above downwards (fig. CXLI. 2). 381. In consequence of this conformation and arrangement of the ribs, every degree of motion which is communicated to them, necessarily influences the capacity of the space they enclose. If they are moved upwards they must enlarge that space at the sides, because the intervals between each other will be increased (fig. CXLI. 2); and from behind forwards, because the distance between the spinal column and the sternum (the sternum being protruded forwards with their cartilaginous extremities) (fig. CXLI. 3, 4), will be increased. If, on the other hand, they are moved downwards, the capacity of the thorax will be proportionally diminished in every direction (fig. CXLI.). [Illustration: Fig. CXLII. 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.