THE HUMAN STERNUM 23 



2 . Growth of the Mesostemum after Birth. 



After the age of twenty-one, the mesosternum is a single bone, and 

 after sixteen years the fusion of its component parts makes it impossible to 

 determine the precise number of the original elements. 



Ridges on the Mesostemum. 



In the adult bone the only evidence of the number of component 

 parts of the mesosternum is derived from the presence of ridges across the 

 bone ; but these ridges are quite unreliable guides. They are variable in 

 number and strength, and are not normally present except between the 

 attachments of the third costal cartilages (69-2 per cent, of all sterna over 

 twenty years). 



Out of five hundred and twenty-four cases (Table VI) the ridges are 

 absent altogether in one hundred and forty cases (26-7 per cent.) There 

 are three hundred and sixty-three examples of a ridge opposite the third 

 costal attachment (69*2 per cent.) ; two hundred and five examples of a 

 ridge opposite the fourth (39*1 per cent.) ; twenty-one of a ridge opposite 

 the fifth (4 per cent.) ; and two of a ridge opposite the sixth costal attach- 

 ment (o*3 per cent.). 



The only evidence which these ridges afford supports the facts of 

 ossification ; there are at least three elements engaged in the formation of 

 the mesosternum. They are of no value in deciding upon the presence or 

 absence of a fourth element ; and there is no justification for the statement" 

 that the mesosternum ' is marked on its anterior surface by three slight 

 transverse elevations at the lines of junction of its four component parts.' 



Ossification of the Mesostemum 

 Number of Elements. 



An examination of one hundred and forty-one mesosterna between 

 the ages of birth and sixteen years confirms the view adopted from an 

 examination of foetal sterna (p. 18), that it is usual to find three centres 

 of ossification, but only exceptionally four centres (Table VII). 



Three centres of ossification are practically constant ; and in two 

 cases only was the third centre absent (at the ages of two and nine years, 



