THE MECHANICS OF RESPIRATION 



333 



converted into an upward movement, which is greatest in that part of the 

 shaft lying parallel to the axis of rotation of the neck (Fig. 111). 



The upper ribs from the first to the fifth form a cone-shaped top to the 

 thorax, whereas the lower ones form a vertical series, each being situated 

 almost directly above its neighbor. The upper set is arranged for the 

 expansion of the conical upper lobe of the lungs, the lower for the ex 

 pansion of the more or less cylindrical lower lobes. During inspiration 

 the anteroposterior diameter of the conical portion of the thorax, in- 

 creases, because the ribs, together with the sternal connections, move 

 through progressively increasing arches, and each lower rib tends to over- 

 ride the rib just above. The maximal rise of the ribs from the first to the 

 tenth during inspiration shifts more and more from the anterior to the 



Fig. 111. A, first dorsal vertebra; B, sixth dorsal vertebra and rib. Axis of rotation shown in 



each case. 



lateral aspects of the thorax, because the angle formed by the shaft near 

 the neck of the rib approaches nearer to the articulating joints on the 

 vertebras. 



An examination of the shape of the first rib, its relationship to adjacent 

 structures and its movements, shows that it differs from the others in 

 its respiratory function. The first pair of ribs and the manubrium sterni 

 are bound closely together by short, wide costal cartilages, and form a 

 structural unit which Keith 1 calls the -thoracic operculum. This lid is 

 articulated behind with the first thoracic vertebra by a joint, which is 

 more nearly transverse than that of the rest of the costal series ; and in 

 front with the manubrium, which is also articulated with the clavicles 



