256 THE MECHANICAL FUNCTIONS. 



vient to the finer senses, namely, those of siglit, of hearing, 

 and of smell. The security which these organs derive from 

 this protection allows of their being carried to a higher de- 

 gree of improvement than could be attained in the lower or- 

 ders. 



There is also another advantage, of considerable moment, 

 which results from the internal situation of the skeleton, 

 namely, that it admits of an indefinite extension by growth, 

 without interfering with the corresponding enlargement of 

 the softer organs; for we have seen that in all the instances 

 in which this arrangement is reversed, that is, whenever the 

 enclosing surfaces become solid, and can no longer yield to 

 the dilatation of the contained organs, no alternative remains 

 but that of breaking up the exterior case, and vv-holly cast- 

 ing it off*, to make room for the farther growth of the ani- 

 mal; after which operation, it has to be replaced by another 

 coverincr of larger dimensions. This operation is generally 

 required to be performed a great number of times, before 

 the animal can acquire the size it is destined to attain. 

 Hence the perpetual moultings of the caterpillar; hence the 

 repeated castings of the shells of the Crustacea; and hence 

 also the successive metamorphoses of the insect. Nothing 

 of this kind takes place among the Vertebrata; where all the 

 organs are developed in regular and harmonious succession, 

 without the slighest mutual interference, and without those 

 vicissitudes of action, and of torpidity, which we witness in 

 the chequered existence of the insect. 



§ 2. Structure and Comjiositioii of the Osseous Fabric. 



The process employed for the formation and extension of 

 the solid frame-work of the Vertebrata diff'ers totally from 

 that which we have seen exemplified in the growth of shells, 

 or of the hard coverings of insects and of crustaceous ani- 

 mals. These latter structures, and the modes adopted for their 

 increase, are suited only to animals in which the functions 

 of the economy have not reached that perfection to w^hich 

 they are carried in the higher classes. In the more elabo- 



