34 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 29. — The eleventh rib of the right side seen front behind (f). 

 Fig. 30. — The twelfth rib of the right side seen from behind (J). 

 Fig. 3 1 . — The sternum seen from in front (£) . 

 Fig. 32. — The sternum seen from the left side (£). 



THE STERNUM. 



The breast-bone, or sternum, is a single, flat, oblong bone. It lies approximately in the 

 frontal plane and forms the middle portion of the anterior wall of the thorax, and by its articu- 

 lation with the clavicles, it completes the shoulder girdle anteriorly. It is composed (Fig. 31) 

 of three distinctly separated portions placed one above the other, an upper broad handle, the 

 manubrium, a middle piece, the body or gladiolus, and a lower piece, the xiphoid process. The 

 three portions are either separated by cartilage or are united by bone; the former condition 

 prevails between the manubrium and the body (synchondrosis sternalis), while the latter obtains 

 between the body and the xiphoid process. At the synchondrosis between the manubrium and 

 the body there is usually quite an obtuse angle, open posteriorly, the angulus sterni (angle of 

 Louis). The sternum is not exactly in the frontal plane, but is placed somewhat obliquelv, 

 so that the upper end is considerably nearer to the vertebral column than the lower one, a rela- 

 tion which is also partly due to the curvature of the vertebral column (see page 14). 



The manubrium is considerably broader than the body of the bone, and is broadest above 

 and narrowest below; its anterior surface is slightly convex, and the posterior surface slightlv 

 concave. The upper margin presents three rounded notches, a median shallow interclavicular 

 or jugular notch, and two lateral deeper clavicular notches, which are covered with cartilage 

 and accommodate the sternal ends of the clavicles. Immediately below each clavicular notch 

 there is a notch (Fig. 32) upon the lateral margin of the sternum for the reception of the broad 

 costal cartilage of the first rib, which is joined to the sternum in this situation by a synchon- 

 drosis, and each side of the lower end of the manubrium presents a demifacet, for articulation 

 with the cartilage of the second rib. 



The body is usually narrowest above, gradually widening as it descends, until it attains 

 its greatest breadth in its lower third, and then rapidly narrowing again as it approaches the 

 xiphoid process; it is occasionally, however, of uniform width throughout. Its nearly flat 

 anterior (ventral) surface is called the planum sternale, and sometimes presents transverse lines 

 (Fig. 34) which indicate the original fusion of several parts situated one above the other. 



At the margins of the body (Fig. 32) are found notches for the cartilages of the six lower 

 true ribs, that for the second rib being situated at the junction of the manubrium and the body, 

 that for the sixth rib on the lower margin of the body, and that for the seventh in the angle 

 between the body and the xiphoid process. The notches for the fifth, sixth, and seventh ribs 

 lie close together, and the fourth notch is situated below the middle of the entire bone. 



The xiphoid process varies greatly in shape and size. It is always markedly narrower 

 than the body, often partly or even wholly cartilaginous, and it is sometimes perforated. The 

 female sternum is usuallv shorter and broader than that of the male. 



