272 PALAEONTOLOGY. 



The skeleton is divided into three distinct parts: the 

 HEAD, the TBTJNK, and the EXTBEMITIES (fig. 182). 



FIG. 182. The Megatherium. 



The head is composed anteriorly of theface, formed of two 

 jaws, of which the lower is moveable ; of the orbital cavities 

 or sockets for the eyes ; and posteriorly of the cranium, or 

 osseous case for lodging and protecting the brain. The 

 trunk is supported by the spinal axis, composed of a series 

 of separate bones called vertebrae, which are moveable 

 on each other, but firmly united together by joints, liga- 

 ments, and a connecting substance. To the central part or 

 body of each vertebra is articulated an arch of bone for 

 lodging and protecting the spinal cord. The first verte- 

 bra carries the skull, and the posterior vertebrae are often 

 prolonged into a tail. To the sides of some of the vertebrae 

 are attached the ribs, which form a cage for lodging and pro- 

 tecting the organs of respiration, circulation, and often those 

 of digestion. The ribs are for the most part united before 

 to a breastbone or sternum. The extremities, when they 

 exist, never exceed two pairs, but the posterior pair, or 

 both, are sometimes absent. 



The Vertebrata are divided into four classes : Fishes, 

 Reptiles, Birds, Mammifera. 



