CONNEXION OF SEPARATE PIECES OP SKELETON. 455 



the parts already formed ; there is nothing like that " inter- 

 stitial " change which we see in bone, and which is performed 

 by the agency of the blood conveyed through the Haversian 

 canals that traverse its substance j and where the skeleton is 

 external, it must either be adapted by such additions to the 

 augmenting bulk of the body it incloses, or must be cast-off 

 and replaced by another. The latter method is that which 

 is followed in the Crustacea ; of the former we have examples 

 in the shell-bearing Mollusks, whose shells receive successive 

 additions at their free margins, and in the Echinodermata, 

 whose box-like envelopes are made to increase equally in all 

 directions, by additions to the edges of the numerous separate 

 pieces of which they are composed ( 118). 



600. The different portions of the skeleton are articulated, 

 or united by joints to one another, in such a manner that 

 they can move with greater or less freedom. This we see 

 both in the Vertebrated and in the Articulated classes. In the 

 latter, the joints are for the most part very simple in their 

 construction. The different rings or pieces are held together 

 by a flexible membrane passing from one to the other j this 

 seems to be little else than a portion of the integument 

 originally covering the body, which has remained uncon- 

 solidated whilst the rest has been hardened. And sometimes 

 they are made to adhere to each other by a kind of " solder- 

 ing," so as to be altogether immovable. But in the internal 

 skeletons of the Vertebrata we find a more complex mode of 

 union, fitted to afford scope for the greater variety of motions 

 which their parts perform. Here, too, we find some parts 

 immovably united to each other, where support and protec- 

 tion alone are required. These immovable articulations, of 

 which there are several kinds, will be first considered. 



601. All the bones of the head and face (with the excep- 

 tion of the lower jaw), in Man and the higher Vertebrata, 

 have their edges in immediate contact with each other; so 

 that they hold together in the dry skull, as well as during 

 life. Those bones of the skull, which inclose and protect the 

 brain, are very firmly united by what are termed sutures, 1 

 which are mostly formed by the interlocking of the jagged 

 edges of one bone into corresponding notches of the adjoining 

 one : though in some this kind of union is incomplete, while 



1 From the Latin sutura, a seam. 



