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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



are drawn upward and outward by the contractions of other inspiratory mus- 

 cles, thus enlarging the thorax in the antero-posterior and lateral diameters. 



Movements of the Ribs. The movements of the ribs during inspiration 

 are, as a whole, essentially rotations upward and outward upon axes which are 

 directed obliquely outward and backward, each axis being directed through 

 the costo-vertebral articulation and a little anterior to the costo-transverse 

 articulation. The vertebral ends of the ribs lie higher than their sternal 

 extremities, so that when the ribs are elevated the anterior ends are advanced 

 forward and upward. The arches of the ribs are inclined downward and 

 outward, and, owing to the obliquity of the axes of rotation, the convexities 

 are rotated upward and outw r ard, or everted. Thus both the antero-posterior 

 and lateral diameters are increased. 



The degree of obliquity of the axes of rotation of the different ribs varies. 

 The axis of the first rib is almost transverse (Fig. 131), while that of each 

 succeeding rib to the ninth, inclusive, becomes more oblique (Fig. 132). The 



Axis of 

 rotation. 



Axis of 

 rotation. 



FIG. 131. First dorsal vertebra and rib. 



FIG. 132. Sixth dorsal vertebra and rib. 



more oblique the axis, the greater the degree of eversion ; consequently the 

 first rib is capable of but slight eversion, while the lower ribs may be everted 

 to a relatively marked extent. Moreover, the peculiarities or the absence of 

 the costo-transverse articulations materially affect the character of the move- 

 ments of the different ribs. Thus, the facets on the transverse processes of the 

 first and second dorsal vertebra? are cup-shaped, and into them are inserted 

 the conical tuberosities of the ribs, thus materially limiting the rotation of the 

 ribs ; while the facets for the articulations of the third to the tenth ribs, inclu- 

 sive, assume a plane character which admits of larger movement. The facets 

 for the third to the fifth ribs are almost vertical, thus allowing a free move- 

 ment upon the oblique axis ; while the facets for the sixth to the ninth ribs, 

 inclusive, are directed obliquely upward and backward, and admit of a move- 



