2 ORGANS OF SUPPORT, 



keep pace with the progress of growth in the living parts, 

 by being periodically cast off and renewed ; or they may in- 

 crease by the addition of more extended layers to their 

 surface ; or their dimensions may be continually influenced 

 by the contact of the parts which formed them. But when 

 this solid frame-work is internal, and is everywhere sur- 

 rounded by the soft parts, giving attachment to muscles, or 

 enveloping and protecting delicate organs, it cannot be con- 

 veniently removed from the system in a mass, nor preserve its 

 proportions by the mechanical addition of layers to its sur- 

 face, and is generally organized or permeated in every point 

 by the soft parts which absorb the decayed materials and re- 

 new them particle by particle. The earthy materials thus 

 formed by animals for the support of their soft parts are 

 various, and their particles are generally united together by 

 means of a condensed albuminous or gelatinous matter, 

 which gives firmness and tenacity to the mass. Silica is 

 found in the lowest forms of radiated animals ; carbonate of 

 lime in the molluscous classes ; carbonate and phosphate 

 of lime in the articulated animals, and phosphate of lime 

 in the organized skeletons of the vertebrata. These earths, in 

 consolidating, assume forms by the influence of laws which are 

 in accordance with their ordinary physical properties, this we 

 observe most obviously in the lowest animals, and least in the 

 highest classes where the crystalline arrangement of the par- 

 ticles is most equivocal ; but under every condition they alike 

 form a normal part of the structure, a solid frame-work 

 more or less complete, constant in its form and structure 

 in the same species, and varying in its form with the speci- 

 fic differences of animals. This solid framework forms the 

 osseous system of animals, or the skeleton, as it has been 

 termed from the dry and earthy nature of the materials 

 which compose it. The osseous system, though not the 

 most important nor the most universal system of animal or- 

 ganization, is met with under some form in every class of the 

 animal kingdom, though not in all the animals of each class. 



