CHAP, ii.] THE SKELETON. 13 



of the lacunae. This structure is readily recognised 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 organisation 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 per- 

 manently in a cartilaginous or fibrous condition. 



The different bones composing the skeleton are con 

 nected together either by sutures, or by movable 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 in- 

 vesting the bones, passing directly from one to the other, 

 permitting 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. 



