34 THE HUMAN STERNUM 



Metastemum. 



Markowski mentions one case in which the metasternum is absent 

 altogether. He states that it begins to ossify just after birth, or in the first 

 three years ; and among his cases the latest date was in the sixth year. 

 He states further that ossification occurs by two lateral or by one median 

 centre. My observations agree more with those of previous observers, 

 who lay stress upon the variability in the time but the stability in the mode 

 of ossification of this part of the bone — from one median centre. 



The general conclusions at which Markowski arrives, and upon 

 which I would lay most stress, are (i) the agreement in our observations 

 that the sternum is ossified from single median centres more often than 

 from bilateral double centres, although he asserts that this is a higher and 

 later mode of development ; and (2) the obvious and striking irregularity 

 in the mode of ossification of the mesosternum. 



Although great stress is laid upon the metameric arrangement of the 

 parts of the sternum, Markowski admits that this is secondary ; and he fails 

 to indicate — just as my specimens fail to indicate — a metameric relation- 

 ship between the lower true ribs and parts of the mesosternum. And 

 although he pins his faith on the association of the presternum with the 

 first and second costal cartilages, he admits that a single median centre is 

 much the commonest for this part of the bone (68*4 per cent.) 



In my opinion, the great value of Markowski's researches is to show 

 again how very variable is the deposition of the centres of ossification. He 

 rightly lays stress upon the asymmetry of centres ; and, more particularly, 

 upon the occurrence of additional accessory centres round the main centres. 



These points, to my mind, are supporting evidence in favour of the 

 view that the centres of ossification are not of morphological value ; but 

 that the sternum is essentially a shield, in whose substance these osseous 

 deposits occur as bosses to strengthen it, not through hereditary influence, 

 but through causes which are mechanical, functional, and adventitious. 



Markowski's memoir is a valuable and thoughtful contribution to 

 the subject, and has been of great interest in the final preparation of the 

 present work. 



