110 THE MECHANICAL EUNCTIONS. 



exterior covering ; as may readily be detected by 

 viewing the surface with a magnifying glass.* 

 After a certain period of the growth of the quill, 

 the pulp ceases to supply the materials for forming 

 the spongy substance which occupies the interior 

 of the quill. But although it no longer secretes, it 

 still retains its place ; and the capsule continuing 

 to deposit horn, the quill becomes a hollow tube of 

 considerable diameter. When it has attained a 

 certain size, the pulp begins to shrink, and the 

 diameter of the tul3e diminishes ; so that it exhibits 

 a tapering form at both ends. Thus mere varia- 

 tions in the bulk and the action of the pulp, accom- 

 panied with changes in that of the capsule, are 

 sufficient to account for every diversity in the form 

 and condition of the resulting structures. 



Among the mechanical uses of the integument, 

 that of serving as a cushion for relieving the more 

 prominent parts of the frame, and especially of the 

 bones, from unequal pressure, ought not to be over- 

 looked. This object is promoted by the interposi- 

 tion of a layer o^ fat, which is another animal 

 substance entitled to be enumerated among the 

 elements of its structure. It consists of an oily 

 fluid, composed according to the analysis of 

 Chevreuil, of two constituent principles, which he 

 has distinguished by the terms stearin and elain.'\ 

 In warm blooded animals the temperature of the 



* It is observed by F. Cuvier, that this striated appearance is 

 peculiar to the quills of porcupines of the old world. Those from 

 America have no such arrangement of laminae. 



t These two constituent principles possess very different degrees 

 of cohesion ; elain being- liquid, and stearin nearly solid, at the usual 

 temperature : and the consistence of the compound will, therefore, 



