150 MAMMALIAN ANATOMY 



triangular flattened process, to the concave caudal margin of which the 

 end of the first costal cartilage is attached. The dorsal surface (Fig. 

 102) is concave on a cephalo-caudal line and convex transversely. A 

 faint groove separates the process from the body of the bone. 



The Eighth Sternebra (Fig. 99) is called the xiphisternum or 

 metasternum. It is longer than the manubrium, and the length 

 is still further increased by a fan-shaped terminal cartilage known as 

 the xiphoid* or ensiform 1 cartilage. The bony portion of the xiphi- 

 sternum is cylindrical, narrow at the caudal end. It is the most 

 movable portion of the sternum, and does not furnish attachment to 

 any of the costal cartilages. It is joined to the seventh sternebra by 

 a quadrate piece of cartilage the sides of which are more or less re- 

 lated to the attachment of the costal cartilages of the eighth and ninth 

 ribs. 



The remaining sternebrse constitute the mesosternum. They de- 

 crease gradually in length until the seventh is reached, which is one- 

 fourth shorter than the second, which in turn is two-fifths shorter 

 than the manubrium. They do not decrease in breadth ; indeed, the 

 fifth, sixth, and seventh are actually wider. In the second and third 

 the ventral surface is distinctly narrowed in front. In the seventh 

 sternebra of adult animals each lateral surface is raised at the caudal 

 end into a process which joins the eighth costal cartilage, the caudal 

 surface presenting on each side of the point of union of the xiphi- 

 sternum a facet for the end of the ninth costal cartilage. 



The length of the entire sternum is twenty times greater than its 

 average width. 



Nomenclature. Sternum is from the Greek sternon, the breast. 

 As used by Homer, it always refers to the male breast, stethos (whence 

 stethoscope) referring to the breast of either sex. Galen employs 

 thorax for the breast, and first used sternon for the breast-bone. 

 Sternum is derived from stereos, hard, because, owing to the absence of 

 muscular covering, it is hard when felt through the skin. 



Muscular Attachments. The sternum affords attachment to the 

 following muscles : the sternomastoid and the sternohyoid, to the 

 manubrium ; the pectoral, to the ventral surface ; the triangularis 

 sterni, to the dorsal surface ; and the diaphragm, to the xiphisternum, 



1 Xiphos (G-r.), ensis, a sword. 



