RESPIRATION. 509 



ment upward and backward as well as a rotation upon the oblique axis. 

 Finally, the eleventh and twelfth ribs (and generally the tenth) have no costo- 

 transverse articulations, allowing a movement backward and forward as well 

 as rotation upon their oblique axes. While, therefore, the movements of the 

 ribs are essentially rotations upward, forward, and outward upon oblique axes 

 directed through the costo-vertebral articulations and a little anterior to the 

 costo-transverse articulation, they are more or less modified by reason of the 

 motion permitted by the nature or the absence of the costo-transverse articu- 

 lations. Thus, the essential character of the movement of the first to the 

 fifth ribs is a rotation upward, forward, and outward ; that of the sixth to 

 the ninth ribs, a rotation upward, forward, and outward combined with a 

 movement upward and backward; that of the tenth and eleventh ribs, a 

 rotation upward, forward, and outward with a rotation backward ; that of 

 the twelfth rib, chiefly a rotation backward and rather downward. The 

 character of the movement of each rib differs somewhat as we pass from 

 the first to the twelfth ribs. 



During forced inspiration the sternum and its attached costal cartilages 

 with their ribs are pulled upward and outward, while the ninth, tenth, 

 eleventh, and twelfth ribs are drawn backward and downward. During 

 expiration these movements are of course reversed. 



The intercostal spaces during inspiration, except the first two, are widened. 1 

 The reason for this opening out must be apparent when we remember that 

 the ribs are arranged in the form of a series of parallel curved bars directed 

 obliquely downward, and the fact may be demonstrated by means of a very sim- 

 ple model (Fig. 133) consisting of a vertical support and two parallel bars, a, 6, 

 placed obliquely. If, after measuring the distance c, d, we 

 raise the bars to a horizontal position, the distance e,f will 

 be found to be greater than c, d t since the bars rotate around 

 fixed points placed in the same vertical line. This widening 

 of the intercostal spaces is readily accomplished because of 

 the elasticity of the costal cartilages. 



The muscles involved in the movements of the ribs 

 during quiet inspiration include the sccdeni, the serrati 

 postici superiores, the levatores costarum longi et breves, and 

 the intercostales externi et intercartilaginei. 



The sccdeni are active in fixing the first and second ribs, lustrate the widening 

 thus establishing, as it were, a firm basis from which the 

 external intercostal muscles may act. The scalenus anticus 

 passes between the tubercles of the transverse processes of the third, fourth, 

 fifth, and sixth cervical vertebrae to the scalene tubercle on the first rib. 

 The scalenus medius passes from the posterior tubercles of the transverse 

 processes of the lower six cervical vertebrae to the upper surface of the 

 first rib, extending from the tubercle to just behind the groove for the 

 subclavian artery. The scalenus posticus passes from the transverse pro- 



.* Ebner : Archivfur Anatomic und Physiologic, Anatomische Abtheilung, 1886, p. 199. 



FIG. 133. Model to il- 



