46 THE HUMAN STERNUM 



examined another series of fifty-six sterna with the costal attachments 

 intact, with the result that forty-five sterna (80-3 per cent.) were found to 

 be normal ; eleven sterna (i9"5 per cent.) were abnormal (Table XXII). 

 Of the eleven abnormal cases, two (3-5 per cent.) are examples of failure 

 of the seventh costal cartilage to reach the sternum, in one case on both 

 sides, in the other case on the left side. The nine remaining cases (16 per 

 cent.) are examples of a junction of the eighth costal cartilage with the 

 sternum on one side or both. In two cases there is a junction with the 

 sternum on both sides ; in two cases the junction is on the left side only ; 

 and in five cases the eighth costal cartilage is attached to the sternum on 

 the right side only. Coupled with the larger scries of younger sterna these 

 examples confirm the conclusions of Cunningham" and others, and showing 

 that the eighth costal cartilage has a greater tendency to become attached 

 to the sternum on the right side than on the left. 



Examination of the attachment of costal cartilages to the sternum 

 indicates (i) an individual variability in the connexion of the ribs with the 

 sternum unrelated to any essential change in the structure of the latter, 

 (2) a greater tendency to detachment and downward displacement on the left 

 side, and (3) a firmer and more extensive attachment on the right side, 

 particularly at the ends of the series. 



4. Asymmetry of Clavicular Facets. 



There are slight differences in the size and concavity of the pre- 

 sternal facets for articulation of the clavicles (Table XX). A more 

 important difference, however, which is to be associated with asymmetry of 

 the sternum, occurs on the level of the two facets. It was found unequal 

 in one hundred and ninety-nine cases (38-9 per cent.), and the left facet 

 is much oftener in a higher position than the right one (left, 145 = 28*5 

 per cent. ; right, 54= 10-3 per cent.) 



This agrees with Birmingham's^ conclusion on this point. He 

 found the right facet more often lower in position than the left, and he 

 associates rightly the vertical depth of the interval between the first two 

 costal cartilages with this condition. On the other hand, it is possible to 

 regard the occurrence of a higher clavicular facet as compensatory for 

 curvature caused, it may be, by the costal attachments. 



