6o 



Anatomy of Skeleton 



from the surface over its whole area except on the depressed xiphoid : this lies in the 

 floor of the infrasternal depression. The angle is particularly prominent and marks the 

 level of the second costal cartilages, from which the succeeding cartilages may be counted 

 down ; but it should be pointed out that in some uncommon cases the prominent angle 

 is opposite the third cartilages, the manubrium including what would otherwise be the 

 first piece of the body. It is therefore advisable if, when examining a chest, the angle 

 gives the impression of being lower than usual, to take care to ascertain that the angle 

 is really of the normal type and has the second cartilages opposite it. The abnormality 

 is well known, and is usually considered a reversion to the arrangement normally present 



OSS 



FIG. 48. Showing variations which may occur in ossification, i is a sternum at birth : 

 observe the double centre for manubrium and the single one (perhaps fused) for 

 second piece of body. This is practically a normal specimen. 2, from a young 

 child, although there is considerable growth of the whole sternum, does not yet 

 seem to have any centres in the last piece and thus is hardly normal. Observe 

 that the bone is shaped still in cartilage with separated centres a condition which 

 remains for some years as the centres extend slowly. When there is more than one 

 centre for the manubrium the added centre appears later than the main one ; this 

 applies also to extra centres found sometimes in or between the sternebra. 



in some of the higher monkeys, but we cannot recognise the conditions that are 

 responsible for its occurrence in certain individuals. 



The morphological position of the sternum, the value of its various parts, and their 

 correspondence with the different portions of the sternal apparatus in the lower 

 vertebrates, are controversial questions on which little light is thrown by the onto- 

 genetic development of the human bone. Its comparative shortness in man is corre- 

 lated with other changes in the thorax resulting from the assumption of the erect 

 position. 



In its earliest state the sternum is represented by two bars of condensed tissue, 

 the sternal plates, which are continuous with the ventral ends of the upper eight ribs. 



