393 



In adult women the costal angle is somewhat less than in adult 

 men (47*^). This results in a somewhat higher position of the sternal 

 end of the clavicle. The muscular development of the shoulder is 

 not so great as in men. Hence the outer end of the clavicle tends 

 to descend more. Therefore, the clavicular angle is small (25" — 26°). 



The relative angles and positions of the clavicle at different 

 ages are shown in Fig. IV. 



The results obtained in the foregoing research give some in- 

 formation regarding mechanical conditions connected with the lowest 

 brachial trunk. 



The greater the descent of the shoulder the further will the 

 nerve trunk have to pass to reach its distribution. Some allowance 



Clavicle. 



Child. 

 Ant. EXTBEMrrr 



I^J-RlB. 



Aduu. 



CniLO 



Adult. 



Fiff. IV. 



Fig. V. 



Fig. IV. Diagram to show the clavicular auglein the child and the adult; and 

 also the descent of the anterior end of the first rib. The clavicular angles shown 

 are of the average number of degrees. The descent of the rib is entirely diagrammatic. 

 The figure is intended to show that as the shoulder sinks it tends to stretch the 

 lowest brachial trunk over the first rib. The rib itself also tilts downwards, and 

 thus in the normal subject nullifies the tendency to stretching of the nerve by 

 allowing it to travel to its distribution with a less marked upward curve. It will 

 be seen that the compensation is normally less marked in women. 



Fig. V. Diagram to show how descent of the first rib in growth from infancy 

 to adult age compensates the stretching effect of the descent of the shoulder on the 

 nerve. The curved black line represents the vertebral column. The straight black 

 lines represent the first rib in childhood and in adult age. The costal angles are 

 given in degrees. C represents the position of the inner end of the clavicle in 

 infancy and in adult age. The black circle N represents the crossing of the first 

 rib by the lowest brachial trunk. It will be seen that while the descent of the 

 shoulder tends to lengthen the course which the nerve must pursue to reach its 

 distribution, the descent of the first rib tends to shorten the course of the nerve 

 by allowing it to pass in more direct manner to its distribution. Thus, in the 

 majority of cases, the descent of the first rib compensates the descent of the shoulder. 



is made for this by the slight sigmoid course of the nerve as it 

 passes over the first rib (4). 



