THE THORAX. ill 



compact bone. The spongy structure predominates at the an- 

 terior extremity, for there the rib is comparatively soft. 



Of the Sternum. 



This bone constitutes the middle front part of the thorax, 

 and, owing to the obliquity of the ribs, has its superior end on 

 a horizontal line with the third vertebra of the back, while its 

 inferior extremity is on a horizontal line with the eleventh dor- 

 sal vertebra. It is also placed in a slanting direction, so that 

 the lower part recedes from the spine much farther than the 

 upper. 



The sternum is oblong, somewhat curved, like a bow, so as 

 to be convex in front, and concave behind. It is divided, in 

 the adult, into three distinct pieces; an upper, middle, and 

 lower, which are held together by cartilage and by ligament ; 

 but not unfrequently in advanced life these pieces are all fused 

 into one, by bony union. The first and middle parts join where 

 the second rib is articulated, and the middle and lower where 

 the seventh rib articulates. At these points there is a well 

 marked transverse ridge, both anteriorly and posteriorly, and 

 between them on the front of the bone, there are other ridges 

 not so strong, indicating the original separation of the bone into 

 several other distinct pieces. The lateral margins of the ster- 

 num are somewhat elevated where the ribs articulate. 



The upper end of the sternum is both thicker and broader 

 than the lower end. Where tke first and second parts join, 

 there is a narrowing of the two : the same occurs where the se- 

 cond and third pieces unite. 



The first or upper bone of the sternum, has an irregular 

 square figure ; it projects somewhat above, and is slightly hol- 

 low below. It is scooped out at the superior margin, and pre- 

 sents a point at each end of the scoop. At the side of the lat- 

 ter is a concave and rounded surface, for articulating with the 

 clavicle ; just below which is a rough surface, for the cartilage of 

 the first rib. The bone diminishes much in breadth from this 

 point, and terminates by a narrow oblong face, joining it to the 

 second piece. At each side of this junction both pieces con- 

 tribute to a fossa for the cartilage of the second rib. 



