GENERAL CHARACTERS OF THE MAMMALIA. 399 



hindermost of the ribs, or all of them, articulate with the trans- 

 verse processes only, and not with the centra at all. 



There are usually no bony pieces uniting the ribs with the 

 sternum or breast-bone in front, as in Birds ; but the so-called 

 " 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 rib and costal cartilage. More rarely, as in the Mono- 

 tremes, 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 poste- 

 rior ribs, which fall short of the breast-bone, being known as 

 the " false " ribs. 



The sternum or breast-bone is formed of several 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 at- 

 tachment of the pectoral muscles, as it is in Birds. The ster- 

 num is primitively composed of three pieces, an anterior piece 

 or prtzsternum, a middle piece or mesosternum, and a posterior 

 piece or xiphisternum. The praesternum 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 

 xiphisiernum is the " xiphoid cartilage " of human anatomy, and 

 it commonly remains throughout life more or less unossified. 

 In the Monotremes there is a T-shaped bone above or in front 

 of the praesternum, but this is probably to be regarded as be- 

 longing to the shoulder-girdle, and as representing the " epi- 

 sternum " or " interclavicle" of the Reptiles. 



The normal number of limbs in the Mammalia is four, two 

 anterior and two posterior ; and hence they are often spoken 

 of as "quadrupeds," though all the limbs are not universally 

 present, and other animals have four limbs as well. The ante- 

 rior limbs are not known to be wanting in any Mammal, but 

 the posterior limbs are absent in the Cetacea and Sirenia. 



As regards the structure of the anterior limb, the chief 

 points to be noticed concern the means by which it is con- 

 nected with the trunk. The scapula or shoulder-blade is never 

 absent, and it is in the form of a broad flat bone, applied to the 



