490 THE VERTEBRATE SKELETON. 



In the GARNI VORA, too, the sternum (fig. 76) is long and 

 narrow and formed of eight or nine pieces, all of nearly the 

 same size. The xiphisternum generally ends in an expanded 

 plate of cartilage. 



In INSECTIVORA the sternum is well developed but variable. 

 The presternum is commonly large and is sometimes as in the 

 Hedgehog (Erinaceus) bilobed in front, sometimes as in the 

 Shrew (Sorex) trilobed. It is especially large in the Mole 

 (Talpa) and is expanded laterally and keeled below. 



In the CHIROPTERA the presternum is strongly keeled and 

 so is sometimes the mesosternum. 



Among PRIMATES, in Man and the Anthropoid Apes the 

 sternum is rather broad and flattened ; the mesosternum 

 consists of four segments which are commonly fused together 

 and the xiphisternum is imperfectly ossified. 



THE RIBS. 



Free ribs are borne as a rule only by the thoracic ver- 

 tebrae ; ribs may be found in other regions, especially the 

 cervical and sacral, but these are almost always ankylosed to 

 the vertebrae. As a general rule the first thoracic rib 

 joins the presternum, while the succeeding ones are attached 

 between the several segments of the mesosternum. Some 

 of the posterior ribs frequently do not reach the sternum; 

 they may then be attached by fibrous tissue to the ribs in 

 front, or may end freely (floating ribs). There are generally 

 thirteen pairs of ribs, and in no case do they have uncinate 

 processes. 



In MONOTREMES (fig. 102, B) each rib is divided not into 

 two but into three parts, an intermediate portion being inter- 

 posed between the vertebral and sternal parts. The sternal 

 ribs are well ossified, and some are very broad and flat. The 

 intermediate portions are unossified, those of the anterior ribs 

 are short and narrow, but they become longer and wider 

 further back. 



