THE HUMAN STERNUM 19 



mesosternum is probably associated with the method of early development 

 of the cartilage, but it is at the same time plain that it is not necessarily 

 coincident with a bilateral ossification of the mesosternum. 



Sternal foramina (Table IV) occur only occasionally (in seventy-two 

 out of two hundred and thirty-six cases, or 30-5 per cent.) The number 

 and situation of the perforations are as follows : — 



One foramen in the metasternum 



Two foramina in the metasternum . 



One foramen in the mesosternum 



Two or more foramina (in the mesosternum 

 and metasternum) .... 



72 ... 30-5 



The great majority thus occur singly in the metasternum, and the pre- 

 sternum is never perforated. From an examination of a human embryonic 

 sternum, 9 mm. in length (third month) (PI. II, Fig. 19), one is led to 

 believe that the perforation is due to the persistence of vessels, preventing 

 the conversion of the embryonic connective tissue into cartilage in the 

 middle line. 



Like the grooves, these foramina are not improbably causally associated 

 with the mode of early development of the sternum, and it is therefore note- 

 worthy that neither grooves nor foramina occur in the presternum. 



Union of Sternal Elements and Costal Cartilages. 



An examination of the mode of union of the parts of the sternum 

 with one another and with the costal cartilages brings out the fact that the 

 connexions are much the same in the earlier and later months of foetal life 

 (Table V). 



The presternum and mesosternum are usually connected together by 

 a fibrous lamina (76*4 per cent.) opposite the attachments of the second costal 

 cartilages. More rarely they are fused together by cartilaginous union 

 (23"6 per cent.). I have only met with one example (in a nine months foetus) 

 of a case of alteration in position of the junction to the point opposite the 

 attachments of the third costal cartilages. 



