RESPIRA TION. 40 1 



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 eosto-vertebral articulations and a little anterior to the 

 costo-trans verse 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. 71) consisting of a vertical support and two parallel bar-, a, b, 

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

 raise the bars to a horizontal position, the distance c,J will 

 be found to be greater than c, d, 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 which may be involved in the movements 

 of the ribs during quiet inspiration include the scaleni, the 

 serrati postid superiores, the levatores eostarum longiet breves, 

 and the intercostales externi ei intercartilaginei. 



The Hcaleni are active in fixing the first and second ribs, iustrat< the widening 



thus <■stal.lisl.ing, as it were, a firm basis from which the '; r,h " i " , " r ' : . 08 a ^ 1 ft s n pace9 

 ~' during Inspiration. 



external intercostal muscles may act. The scalenus anticus 

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

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

 The scalenus medius passes from the posterior tubercles of the transverse 

 processes of the lower six cervical vertebra 1 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- 



1 Elmer: Arehivjur Anatomie vmd Physiologic, Anatomiscbe Abtheilung, L886, 8. L99. 

 Vol. I.— 2fi 



11. Model to [1- 



