392 



rib reaches a somewhat greater obliquity in adult males than in adult 

 females. Probably the rectus abdominis is a powerful factor in 

 producing and maintaining this obliquity. 



From a consideration of the clavicular and costal angles it is 

 evident that the similarity between the position of the clavicle in 

 the child and in the woman is apparent rather than real. 



In considering the relative position of the shoulder at different 

 ages and its descent during extra-uterine life, one must remember 

 that each end of the bone alters its position without reference to the 

 opposite end. In the descent of the shoulder, which is simply the 

 descent of intra-uterine life continued after birth, the outer end of 

 the clavicle alters its position and tends to occupy a loAver plane. 



Powerful muscular develop- 

 ment of the muscles of the 

 shoulder girdle will tend to 

 prevent descent of the outer 

 end of the clavicle, while poor 

 muscular development tends to 

 increase the descent. 



The inner end of the 

 clavicle alters its position by 

 passively following the descent 

 of the thoracic operculum. 



In the infant the tonic 

 action of the rectus has not yet 

 come into play. The costal 

 angle is relatively small (30") 

 and consequently the inner end 

 of the clavicle is comparatively 

 high. Full descent of the shoulder has not yet occurred aud thus the 

 outer end of the clavicle also lies comparatively high. 



The result is that the clavicular angle is relatively small (23**). 

 As the child grows, the forces acting on each extremity of the 

 clavicle tend to cause it to take up a relatively lower position. 



In the adult male the costal angle is large (50**) and conse- 

 quently the inner end of the clavicle lies comparatively low. The 

 muscular development of the shoulder militates against the descent 

 of the outer end of the clavicle, which is therefore comparatively 

 high in position. Hence the clavicular angle in men is large (30"). 



Fig. III. Diagram to show the costal 

 angle: This is the angle which the first rib 

 makes with the horizontal. 



