26 THE HUMAN STERNUM 



Of eleven sterna at the age of eighteen years, four possess mesosterna (36-3 per cent.) 

 in the form of a single bone ; in one case all the elements are separate ; in six cases the first 

 piece only is separate, and in one of the six it is fused with the presternum. 



Among seven sterna at seventeen years, there are five in which the mesosternal pieces 

 are all separate ; and two in which it consists of two pieces (i " 2 +3+4, and i + 2 ' 3). 



Of sixteen sterna at sixteen years, six have mesosterna, the elements of which are 

 all separate ; five in which they are all fused (31 "a per cent.) ; and five in which the meso- 

 sternum is in two parts ; of the last-named series, four show a cleavage between the first and 

 second, and one a cleavage between the second and third pieces of the bone. 



Out of seven sterna at fifteen years, the mesosternal elements are all separate in three 

 cases, all fused in two cases (28*5 per cent.) In two cases the mesosternum is composed of 

 three separate pieces. 



Of seven sterna at fourteen years, the mesosternal elements are all separate in three 

 cases ; all fused in one case (i4'2 per cent.) In two cases the first piece is separate ; and in 

 one case the first and fourth pieces are separate, the second and third being fused together. 



In a sternum of thirteen years of age, the parts of the mesosternum are all separate. 



Of five sterna at twelve years, the elements of the mesosternum are all fused in one 

 case (zo per cent.) ; and all separate in another. In two other cases, the first, and the first 

 and second, respectively, are separate from the rest. 



Among three mesosterna at eleven years, there are two pieces in two cases (i ' 2 + 3 : 

 I • 2 + 3 + 4) ; and in one case three pieces (i + 2 ■ 3 • 4). 



Three sterna at ten years, and six at nine years, show all the mesosternal elements 

 separate. 



Of three cases at eight years, the mesosternal elements are all separate in one case ; in 

 two cases the first two elements are separate (i ■ 2 ■ 3 + 4). 



Among five mesosterna at six years, different conditions are present in each case : — 



Among five sterna at five years, all the mesosternal elements are separate in four cases ; 

 in one case the second and third pieces are fused together. 



Of three sterna at four years, one has all the mesosternal elements separate ; in two 

 instances the third and fourth pieces are fused (1-2-3 + 4). 



In eighteen sterna, aged o — z years, the mesosternal elements are separate in all. 



This statement has been made at length to show that although there 

 is wide variability, the examination of an extended series shows a considerable 



