CHAPTER II. 



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 

 regards those organs and tissues, or may be more or less 

 embedded in enveloping softer structures. In the former 

 case it is called an Exoskdeton, 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- 

 terised by a peculiar histological structure and chemical 

 composition, being formed mainly of a gelatinous basis, 

 strongly impregnated with calcium phosphates and car- 

 bonates, and disposed in a definite manner, containing 

 numerous minute nucleated spaces or cavities called 

 lacuna, connected together by delicate channels called 

 canaliculi, which radiate in all directions from the sides 



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

 mals which have an ossified exoskeleton. 



