327 



Reasoning from their individual statistics some authors have ad- 

 vanced theories to account for the position of this 8th sternal rib. 

 The most important of these are the theories of Cunningham (2) and 

 Tredgold (4). Cunningham, from the fact that in his series the con- 

 dition was most frequent in males and on the right side put forward 

 the view that it was associated with right handedness in man. Tred- 

 gold again was of opinion that the predominance of the condition on 

 the right side was related to the greater functional importance of the 

 organs on that side — the additional sternal rib being of a protective 

 nature. 



In the extended series however it is apparent that the difference 

 between the two sides (about 47o) is too slight to be explained on 

 such general hypotheses and certainly the figures afford little evidence 

 in support of either of the preceding theories. 



The condition is interesting chiefly in view of the generally ac- 

 cepted opinion that a degeneration at the caudal end of the thorax 

 is in progression. The number of sternal ribs varies greatly in the 

 different mammalian orders but if we confine our attention to the 

 order Primates we find that a degeneration of the sternal ribs takes 

 place in fairly regular order from the lowest to the highest forms- 

 Tredgold (4) and Keith (6) have each carefully investigated the 

 arrangement of the ribs in monkeys and their results generally are in 

 accordance. According to Tredgold (4), in the Lemuridae and Platyr- 

 rhini there are usually more than 8 sternal ribs — in Cercopitheci 

 and Macaci it is practically the rule to find 8 sternal ribs — while 

 in Chimpanzees, Gorillas and Gibbons the presence of an 8th sternal 

 rib may be expected in 54°/o, 28*^/0 and 18 7o of cases respectively. 

 In the orang an 8th sternal rib is present only in very rare cases. 



The 7th rib in man normally reaches the sternum but its mode 

 of attachment is subject to considerable variation. The following are 

 the conditions observed in the 51 cases examined: 



7th rib has a lateral attachment to the metasternum 

 7th rib is attached in front of the metasternum . . 

 7th ribs are articulated with one another in front . 

 7th ribs are fused with one another in front . . . 

 7th ribs fail to reach the sternum 



Right Left 



21 24 



15 13 



12 12 



1 1 



2 1 



51 51 



From these figures it will be seen that in a large percentage of 

 cases (56"/o right, 52% left) the 7th costal cartilages have an attach- 

 ment distinctly anterior to the plane of the metasternum. 



