364 THE MECHANICAL FUNCTIONS. 



tection. The solid mass of bone, thus provided 

 to defend the brain, gives also the opportunity 

 of lodging safely the delicate apparatus subser- 

 vient to the finer senses, namely, those of sight, 

 of hearing, and of smell. The security which 

 these organs derive from this protection allows 

 of their being carried to a higher degree of im- 

 provement than could be attained in the lower 

 orders. 



There is also another advantage, of consider- 

 able moment, which results from the internal 

 situation of the skeleton, namely, that it admits 

 of an indefinite extension by growth, without in- 

 terfering 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 wholly 

 casting it off, to make room for the farther 

 growth of the animal ; after which operation, it 

 has to be replaced by another covering of larger 

 dimensions. This operation is generally re- 

 quired to be performed a great number of times, 

 before the animal can acquire the size it is 

 destined to attain. Hence the perpetual moult- 

 ings of the caterpillar ; hence the repeated 

 castings of the shells of the Crustacea ; and 

 hence also the successive metamorphoses of the 



