122 THE MECHANICAL FUNCTIONS. 



shows these converging laminae, which being 

 of a dark colour, give to the surface of the 

 quill the appearance of being grooved ; this, 

 however, is merely an optical illusion, occasioned 

 by the dark laminae being seen through the 

 transparent exterior covering ; as may readily be 

 detected by viewing the surface with a magni- 

 fying glass.* After a certain period of the growth 

 of the quill, the pulp ceases to supply the ma- 

 terials 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 tube diminishes ; so that it exhibits a taper- 

 ing form at both ends. Thus mere variations in 

 the bulk and the action of the pulp, accompanied 

 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 especi- 

 ally of the bones, from unequal pressure, ought 



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

 peculiar to the quills of porcupines of the okl world. Tliose from 

 America have no such arrangement of laminae. 



