1 8 THE HUMAN STERNUM 



The presternum is generally formed by a single centre (79 per cent.) 

 Where there are two centres, they are, generally, vertically placed. Three 

 or more centres occur quite rarely (2-6 per cent.) 



The first centre for the mesosternum is, as a rule, also single 

 (78-5 per cent.) ; when there are two, they are usually laterally placed. 

 The second and third centres of the mesosternum are less frequently 

 single than the above-named centres {S9"i P^"" '^^^^- ' ^°'5 P^^ cent.) 

 When there are two centres, they are in most cases laterally placed. The 

 fourth centre in the mesosternum is not generally present (26 per cent.) 

 When it occurs it is nearly as often double as single ; and when 

 two centres are present, they are as often vertical as lateral in position. It 

 is of course possible, as Markowski'*^ infers, that the lower of two vertical 

 centres represents the metameric centre of ossification for the fifth piece of 

 the sternum ; but, as it usually occurs in the interval between the attachments 

 of the fifth and sixth pairs of costal cartilages, and not between the attach- 

 ment of the sixth and seventh pairs, it has been looked upon as pertaining 

 to that, and not to a lower segment of the bone. 



The part of the sternum between the attachments of the sixth and 

 seventh costal cartilages is cartilaginous at birth. It appears to be ossified, 

 as a rule, by extension from the third or fourth piece, though there is reason 

 for believing that in some rare cases it may possess a separate and indepen- 

 dent centre (Markowski). 



Fissures : Grooves : Foramina. 



One remarkable negative feature characterizes the series of foetal 

 sterna examined. In all there is a complete median plate of cartilage, 

 grooved it may be on one or both sides, perforated in some instances by 

 one or more holes, but in no single instance is there a ;lfj«r<? j/^r«/. A longi- 

 tudinal groove (Table III) is characteristic of the foetal mesosternum, 

 occurring in one hundred and fifty-eight out of two hundred and twenty- 

 two examples, or 70- 1 per cent., a proportion which obtains generally for 

 all ages. It may be faint or obvious ; it may be found on one or both 

 aspects of the mesosternum, and implicate it in the whole or part of its 

 length, but it never appears in the presternum. This grooving of the 



