5(> ORGANS OF SUPPORT, 



FIFTH SECTION. 



Organs of Support in the Spini- Cerebrated or Vertebrated 



Classes. 



In the lowest vertebrated animals we still find traces 

 of the external unorganized shells of inferior classes, in 

 the form of calcareous scales in fishes, and of horny 

 plates in many reptiles ; but these are generally reduced 

 to small detached pieces, and do not serve as organs of 

 support. The organs of support in the vertebrated classes 

 are placed within the soft parts, so that these animals 

 are more intimately related to the properties of surround- 

 ing objects and outward nature, by the sensibility and 

 delicacy of their surface. Their skeleton being internal, 

 it is not exuviable in a mass, and as it cannot grow 

 and preserve its proportions by the simple addition of 

 layers to its surface, it is organized or permeated in all 

 directions by vessels which take "away and replace its 

 materials atom by atom. The phosphate of lime, which 

 forms the chief consolidating earth, increases in its pro- 

 portion to the gelatin as we ascend through the verte- 

 brated classes ; so that the bones of the lowest fishes 

 are soft, flexible, and cartilaginous ; those of hot-blooded 

 classes are of great density and strength, and those of 

 reptiles possess intermediate properties. The bones have 

 a fibrous structure, which is the best adapted for the 

 transmission of minute vessels through their texture. 

 They form solid levers for the motions of the body, 

 and cavities to protect its viscera. The most constant 

 and the first formed part of the skeleton is the verte- 

 bral column, which is composed of moveable vertebrae, 

 each of which consists of several elements that are found 

 most isolated and distinct in the lowest classes, and in 

 the embryo state of the highest. The elements which 



