54O MANUAL OF ZOOLOGY. 



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

 Monotr ernes, 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 bur- 

 rowing 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 

 mesostermim, and a posterior piece or xiphisternum. The 

 pnesternum is the "manubrium sterni" of human anatomy, and 

 is the portion of the sternum which lies in front of the attach- 

 ment of the second pair of ribs. All the other ribs are con- 

 nected with the mesosternum. The xiphisternum is the 

 "xiphoid cartilage" of human anatomy, and it commonly 

 remains throughout life more or less unossified. In the Mono- 

 tremes 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 articu- 

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

 trematd). 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- 



