THE HUMAN STERNUM 21 



of the third costal cartilages. A few instances are found in which one or 

 more costal cartilages (usually the fifth, sixth, or seventh) force their way- 

 through the cartilaginous sternum to the middle line. Absence of the meta- 

 sternum, asymmetry in the attachment of costal cartilages, and fusion of 

 contiguous costal cartilages together, also occur in a few examples. 



Suprasternal Cartilages. 



In two cases suprasternal cartilages have been found, situated at the 

 cephalic border of the presternum, between the clavicular articular surfaces. 

 In one case the cartilages are separated from the presternum and fused 

 together ; in the other case, they are separate from one another, but con- 

 tinuous with the cartilaginous presternum (PI. VI, Figs. 43A, b). 



{b) Ossification of the Human Sternum After Birth 



I . Growth of the Presternum after Birth . 



After the age of sixteen years the presternum is always a single median 

 bone. Among younger sterna, between the first and sixteenth years inclusive, 

 out of eighty-four examples, sixty-nine (82*1 per cent.) possess single median 

 centres ; eleven (13 per cent.) have double vertical centres (eight fused, 

 three separate) ; two cases present double centres, lateral or oblique ; and 

 in two cases three and five centres, respectively, are present. 



These statistics harmonize with the conditions obtaining in the foetal 

 presternum, in which there is commonly a single median centre (79 per 

 cent.), and more often a pair of median, vertical centres (13 per cent.) than 

 lateral or oblique centres (2-3 per cent.) The larger percentage of single 

 median centres in the presternum after birth is doubtless due to the previous 

 fusion of two centres in some cases. 



Completion of the Presternum. 



After the age of sixteen years, there is a still further growth of the 

 presternum by means of an ectochondral ossification around and along the 

 first costal cartilage (PI. V, Fig. 36). The ossification is at first independ- 

 ent of the presternum, but the bone produced soon completely fuses with 



