30 THE HUMAN STERNUM 



2 . Union of the Mesosternum and Metasternum. 



This, though not normal, is a condition essentially associated with 

 advancing age (Table X). It may occur at any time after the growth of the 

 mesosternum is completed. After the age of twenty years, out of five 

 hundred and twenty-one cases, 65'7 per cent, show the metasternum separate ; 

 34 per cent, show fusion. Between fifty and eighty-two years separation 

 occurs in 53-9 per cent ; fusion in 45-3 per cent. After sixty years, union 

 of the metasternum is more frequent than separation (thirty to twenty-five) ; 

 and of thirteen sterna between the ages of seventy and eighty-two, nine 

 show fusion and four show separation of the mesosternum and metasternum. 

 This statement confirms the view that fusion of the metasternum is a senile 

 change — occurring, as it does, after the age of sixty. 



{d^ Summary. 



Reviewing the facts detailed above relating to the ossification of the 

 sternum : the bone is laid down, primarily in association with the shoulder- 

 girdle, as a median cartilaginous lamina, modelled in the form of the future 

 bone, and connected in a definite and symmetrical way with the cartilages 

 of a certain number of ribs. The conversion of the cartilaginous model into 

 the osseous sternum is by endochondral ossification, as in the case of the 

 vertebral bodies, basis cranii, and the epiphyses of the long bones. It is a 

 slow process, and allows of expansion of the bone in all directions in relation 

 to the growth of associated parts. 



We need not discuss the question of the significance of the mode of 

 ossification, or the value to be attached to it in a morphological sense. One 

 does not attach any special morphological significance to the occurrence of 

 'special centres' in the cartilaginous sternum — or, indeed, in any cartilaginous 

 mass in which this method of ossification occurs. In cases of endochondral 

 ossification the deposition of bony ' centres ' appears to be determined by 

 mechanical causes, more or less obscure. One inclines to the belief that 

 bone production may be excited by stress or strain afl^ecting particular points 

 in the cartilaginous mass, causing vascularization and ossification. Reference 

 has already been made to the difference in ossification of the lower end of 

 the femur and the lower end of the humerus. In the former only one 

 epiphysial endochondral ' centre ' appears ; in the latter there are four. It 



