8 THE SKELETON. [CHAP. 



which radiate in all directions from the sides of the lacunas. 

 This structure is readily recognized when a thin section of 

 bone is examined under a moderately high magnifying power. 



Parts composed of bone are, of all the tissues of the body 

 (with the exception of the teeth), the most imperishable, 

 often retaining their exact form and intimate structure ages 

 after every trace of all other portions of the organization has 

 completely disappeared; and thus in the .case of extinct 

 animals affording the only means of attaining a knowledge 

 of their characters and affinities. 



It must, however, be remembered that, at one period of 

 life, the parts composing the skeleton exist in a fibrous or 

 a cartilaginous form, that their transformation into bone is 

 a subsequent and gradual process, and that even in the 

 Mammalia, though in a less degree than in some of the 

 other Vertebrata, the whole of the internal skeletal system 

 is never entirely osseous, but that portions may remain 

 permanently in a cartilaginous or fibrous condition. 



The different bones composing the skeleton are con- 

 nected together either by sutures, or by moveable joints 

 or articulations. 



In the first, the edges of the bones are in close contact, 

 often interlocking by means of projections of one bone 

 fitting into corresponding depressions of the other, and are 

 held together by the periosteum, or fibrous membrane invest- 

 ing the bones, passing directly from one to the other, per- 

 mitting no motion, beyond, perhaps, a slight yielding to 

 external pressure. The bones of the cranium are connected 

 together in this manner. In old animals there is a great 

 tendency for such bones to become joined together by 

 the extension of ossification from one to the other and 

 consequent obliteration of the suture. This process is 

 called synostosis. 



