CHAPTER I L 



THE SKELETON. 



The term Skeleton, in its widest sense, is used to denote a 

 system of hard parts forming a framework which supports 

 or protects the softer organs and tissues of the body," and 

 which may be either entirely external or superficial as re- 

 gards those organs and tissues, or may be more or less . 

 embedded in enveloping softer structures. In the former 

 case it is called an Exoskeleton, in the latter an Endoskeleton. 



It is of the Endoskeleton alone that this work proposes 

 to treat, as in the class Mammalia the external skeleton, 

 when it exists, performs a relatively subordinate part in the 

 economy. 1 



The branch of anatomy called Osteology is commonly 

 restricted to a study of such parts of the endoskeleton as 

 are composed of bony or osseous tissue, a tissue charac- 

 terized by a peculiar histological structure and chemical 

 composition, being formed mainly of a gelatinous basis, 

 strongly impregnated with phosphate and carbonate of 

 lime, and disposed in a definite manner, containing nume- 

 rous minute nucleated spaces or cavities called lacimce, 

 Pjnnected together by delicate channels called canaliculi, 



The» Armadillos and their extinct allies are the only known mam- 

 uils which have an ossified exoskeleton. 



