CONNEXION OP SEPARATE PIECES OF SKELETON. 455 
the parts already formed; there is nothing like that “ inter¬ 
stitial ” change which we see in hone, and which is performed 
by the agency of the blood conveyed through the Haversian 
canals that traverse its substance; 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 Yertebrated 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; 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 Yertebrata 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 Yertebrata, 
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. 
