642 



MANUAL OF ZOOLOGY. 



"sternal ribs" of Aves are represented by the "costal carti- 

 lages " of the Mammals. In some cases, however, the carti- 

 lages of the ribs do become ossified and constitute sternal ribs. 

 Sometimes, as in the Armadillos, there is a joint between the 

 vertebral ribs and sternal ribs. More rarely, as in the Mono- 

 tremes (fig. 357, D), an intermediate piece is found between 

 the vertebral and costal portions of the rib. .Only the anterior 

 ribs reach the sternum, and these are 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. 353) is formed of several 



--/ 



<y 



ig- 353- A, Sternum of Man, with the costal cartilages. B, Sternum and costal 

 cartilages of the Dog : / Praesternum ; m Mesosternum ; x Xiphisternum. 



pieces placed one behind the other, but usually anchylosed 

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

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

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

 narrow bone, but in the Cetacea it is broad. It is only in some 

 burrowing animals (such as the 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 primitively composed of three 

 pieces, an anterior piece or prcesternum, a middle piece or 

 mesosternum, and a posterior piece or xiphisternum. The 



