THE STERNUM : ITS EAELY DEVELOPMENT AND OSSIFICATION. 29 



early development of the cartilage, l3ut it is at the same time 

 plain that it is not necessarily coincident with a bilateral ossifi- 

 cation of the meso-sternum. 



Sternal foramina (Table IV.) occur only occasionally (in 72 

 out of 236 cases, or 30-5 per cent.). The number and situation 

 of the perforations are as follows : — 



One foramen in the meta-sternum 

 Two foramina in the meta-sternum 

 One foramen in the meso-sternum 

 Two or more foramina (in the meso-sternum 

 and meta-sternum) .... 



72 ... 30-5 



The great majority thus occur singly in the meta-sternum, and 

 the pre-sternum is never perforated. From an examination 

 of a human embryonic sternum 9 mm. in length (third month) 

 (fig. 19), one is led to believe that the perforation is due to the 

 persistence of vessels, preventing the conversion of the embry- 

 onic 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 noteworthy that neither grooves nor foramina 

 occur in the pre-sternum. 



(D) 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 connections are much the same in the earlier and 

 later months of foetal life (Table V.). 



The pre-sternum and meso-sternum are usually connected 

 together by a fibrous lamina (76-4 per cent.) opposite the attach- 

 ments 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 9 months foetus) of a case of 

 alteration in position of the junction to the point opposite the 

 attachments of the third costal cartilages. 



The meso-sternum and meta-sternum are usually (77"9 per 

 cent.) united by cartilage, and only exceptionally (221 per cent.) 

 by a fibrous lamina. 



The costal cartilages in the centre of the series are generally 



