4/4 MANUAL OF ZOOLOGY. 



vertebral rib and costal cartilage. More rarely, as in the 

 Monotremes, an intermediate piece is found between the verte- 

 bral 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 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 ani- 

 mals (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 prasternum, 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 meso- 

 sternum. The xiphisternum 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 belonging to the shoulder-girdle, 

 and as representing the " episternum " 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 connected 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 outer 

 aspect of the ribs, and much more developed than in the Birds. 

 The coracoid bone, which forms such a marked feature in the 

 scapular arch of Aves, is fused with the scapula, and only arti- 

 culates with the sternum in the Duck-mole and Echidna (Mono- 

 tremata). In all other Mammals the coracoid forms merely a 

 process of the scapula, and does not reach the top of the breast- 

 bone. The collar-bones or clavicles never unite in any Mam- 



