276 



MAMMALIA. 



called the " true " ribs ; the posterior ribs, which fall short 

 of the breast-bone, being known as the " false " ribs. 



The sternum or breast-bone (fig. 594) is formed of several 

 pieces placed one behind the other, but usually anchylosed 

 together to form a single bone. It is placed upon the ven- 

 tral surface of the body, and is united with the vertebral 



Fig. 594.— A, Sternum of Man, with the costal cartilages, b, Sternum and costal cartilages 

 of the Dog: p, Prsesternum ; m, Mesosternum ; x, Xiphisternuni. 



column by the ribs and their cartilages. It is generally a 

 Ions and narrow bone, but in the Cctacea it is broad. It is 

 only in some burrowing animals (such as tlie Moles) and in 

 the true flying Mammals (the Bats), that the sternum is 

 provided with any ridge or keel for the attachment of the 

 pectoral muscles, as it is in Birds. The sternum is primi- 

 tively composed of three pieces, an anterior piece or prmstcr- 

 num, a middle piece or mesostcrmmi, and a j^osterior piece or 

 xiphistcrnum. The presternum is the " manubrium sterni " 

 of human anatomy, and is the portion of the sternum which 

 lies in front of the attachment of the second pair of ribs. 

 All the other ribs are connected with the mesosternum. 

 The xipliisternum is the " xiphoid cartilage " of liuman 

 anatomy, and it commonly remains throughout life more 



