Thorax 61 



At this time the ribs are very short, so that the sternal plates are on the ventro-lateral 

 aspects of the thorax and separated by a wide interval ; the clavicles reach the top of 

 each plate and become connected by a dense " episternal bar," which can therefore be 

 looked on as connecting the two sternal plates as well as the clavicles. The sternal 

 plates come together at their cranial ends about the sixth week, and fuse in a caudal 

 direction. The episternal bar is probably partly included between the cranial ends of 

 the plates, and partly remains as the interclavicular ligament. 



As the plates come together chondrification extends into them from the ribs, so 

 that the single sternum is at one stage represented by paired cartilages connected by 

 dense mesenchyme. It is necessary to state that some observers maintain that the 

 sternum arises independently of the ribs. 



The mode of origin just described suggests that a presternal structure has been 

 included in the manubrium, but the nature of such a structure remains doubtful : 

 it is interesting, however, to observe that the ossification of this part of the bone is 

 very irregular as to its number of centres, supporting the view that the manubrium 

 is of a compound nature. 



The two plates, joined mesially, form the cartilaginous sternum, but their caudal 

 ends do not fuse, remaining as the cartilaginous costal arch. The xiphoid process is a 

 secondary growth backwards from the end of the fused portion. Junction may be 

 incomplete in places, especially in the lower part, so that when ossification occurs 

 there is a corresponding failure in bone formation and a foramen remains in the bone, 

 a condition not uncommon : the xiphoid is frequently perforated. 



Ossification begins in the sixth month in the upper end of the cartilaginous sternum 

 and progresses from before backwards by the formation of successive centres in the 

 various segments. 



Centre or centres for the manubrium appear sixth month. 

 ,, first piece of body seventh month. 



,, ,, second eighth month. 



third ninth month. 



,, fourth during first year. 



Thus at birth there are centres present (Fig. 48) above the last segment of the 

 body, but not in that segment. 



The centre for the xiphoid appears several years later. 



The number of centres for the segments is variable as a rule one (but often more) 

 for the manubrium, one for the first sternebra, and two for each of the others placed side 

 by side. A single one for the xiphoid. 



The course of ossification just described is liable to frequent changes. Beyond 

 the variation in number of the centres for the manubrium, perhaps the most common 

 modification is an earlier appearance of the body centres, so that the last segment 

 has centres at birth, the occurrence of that for the first segment, perhaps, even preceding 

 the centres for the manubrium. 



Fusion between the sternebrce occurs irregularly, generally progressing from below 

 upwards, occasionally asymmetrically, commencing some years before puberty and 

 being completed about twenty-five. Before it is completed a thin epiphysial plate 

 usually appears for the clavicular facet, and fuses quickly. 



The bony ensiform usually unites with the body in middle life, whereas the cartilage 

 of the angle does not ossify, or only does so in advanced years. 



