THE STKRM-M 



10/ 



cartilaginous bars which in the early embryo represent the ribs. At first these 

 bars fuse together laterally, and for some time the sternum is represented by two 

 strips of cartilage separated by a median fissure. Very early this gap is bridged 

 over anteriorly. Nine costal cartilages are in relation with the sternum at this 

 stage. Gradually the lateral strips unite with each other to form the mesosternum. 

 The ninth costal cartilage divides: one part remains attached to the sternum and 

 becomes the xiphoid, Avhilst the end still attached to the rib acquires a new attach- 

 ment to the eighth cartilage. The ends, still adherent to the sternum, may remain 

 separate and give rise to a bifid metasternum (xiphoid); much more frequently 

 they unite, leaving a small foramen. The eighth cartilage may retain its sternal 

 attachment permanently. 



At first the sternum and costal cartilages are continuous; a Joint soon forms 

 between the pre-sternum and mesosternvim. Gradually joints arise between the 

 costal cartilages and the sternum (except in the case of the first). The division 

 of the mesosternum into sternebrse is a still later process, and arises during the 

 process of ossification. 



Ossification. — The transformation of the sternum into bone is a slow and 

 irregular process. The pre-sternum (manubrium) has a mesial nucleus about the 

 sixth month of intra-uterine life; later, several smaller accessory centres may 



Fig. 115. — Posteeior Surface of the Man'ubrium (Pre-sterxum), with Sternal Exds 

 OF Clavicle and the First Costal Cartilage. 



Sterno-hyoid 



Sterno-thyroid 



appear. The mesosternum usually ossifies from seven centres. The second sterne- 

 bra ossifies from a single median nucleus about the eighth month. Below this, 

 three pairs of ossific nuclei appear, and they may remain long separate. Of these, 

 two pairs for the third and fourth sternebrae are visible at birth. The pair for the 

 fifth sternebra make their appearance towards the end of the first year. The 

 various lateral centres unite in pairs, and at the sixth year the sternum consists of 

 six sternebrie, the lowest (metasternum) being cartilaginous. Gradually the four 

 pieces re])resenting the mesosternum fuse with one another, and at twenty-five they 

 form a single piece, but exhibit, even in advanced life, traces of their original 

 separation. 



The metasternum is always imperfectly ossified, and does not ankylose with the 

 mesosternum till after middle life. The pre-sternum and mesosternum rarely fuse. 

 The dates given above for the various nuclei are merely approximate, for they 

 are extremely variable, not only in appearing, but in their number. The same 

 remark ajjplies also to the age at which the various segments ankylose; hence the 

 sternum affords very uncertain data as to age. 



Abnormalities of the sternum. — The mode of development of the sternum as 

 described above is of importance in connection with some deviations to which it is 

 occasionally sul)ject. At an early jjcriod it consists of two lateral halves; in some 

 rare instances these have failed to unite, and thus give rise to the anomaly of a 



