408 PHYSIOLOGY CHAP. 



two last ribs are loose, and are of less importance in the respiratory 

 mechanism (Fig. 173). 



By means of a double articulation with two vertebral bodies 

 and with a transverse process, each rib is able to rotate round an 

 approximately horizontal axis, which passes along its neck, and 

 forms a greater or lesser angle with the horizontal plane. The 

 axes of rotation of the two corresponding ribs are convergent in 

 front, and decussate, forming angles that decrease, according to 

 Volkmann, from the first to the tenth rib (from 125-88). It 

 follows that in the upward rotation of the ribs, the point of the 

 costal arch which rises most, corresponds not with the anterior end 

 of the rib, but with a lateral point of the costal convexity, through 

 which passes a tangent parallel to the axis of rotation, as shown in 

 Fig. 174. And since the angle formed by the anterior crossing of 

 ^ Q u , the axes of rota- 



tion diminishes from 

 above downwards, 

 the point of maxi- 

 . mal rise' for the 

 / different ribs in in- 

 spiration is displaced 

 and pushed more 

 towards the side, in 

 proportion with the 

 descent from the 

 first to the tenth 

 rib. 



FIG. 174. Horizontal projection of costal ring formed by 5th ribs. lhe increase 111 



(Luciani.) ub and a'b' are the two axes of rotation of the nKlinnp fransMrarao 



double costo- vertebral articulation, which cross, forming in L im l ut! > irdllBVeise, 



this case an angle of 70. The tangent c', parallel to a'b', shows and anterO-pOSteriOr 



the point of greatest elevation of the arch in inspiration. j . , r . 



diameter also varies 



in the different costal hoops, becoming greater in proportion to the 

 inclination of the ribs, and to their length and curvature. 



With the rise and forward inclination of the ribs the sternum 

 is also displaced, describing an arc of a circle from above down- 

 wards, and from behind forwards, in the vertical sagittal plane of 

 the thorax. Since this forward and upward displacement of the 

 sternum is least at its upper, and greatest at its lower end, it 

 follows that the different sections of the sternum must bend a little 

 towards each other, and the costal cartilages make a slight revolution 

 round their longitudinal axes. 



In consequence of the inclination of the ribs and their elasticity, 

 the curvature increases when they rise to an approximately 

 horizontal position; on the other hand, at the junctions of their 

 anterior ends with the cartilages, the curvature becomes slightly 

 flattened, which induces a certain widening of the intercostal spaces, 

 proportional to the degree of their inclinations and respiratory 



