392 THE MECHANICAL FUNCTIONS. 



a little finger. The degree of development of these bones 

 varies in different tribes of birds. 



Feathers are attached to all these divisions of the limb, 

 namely, to the humerus, the fore arm, the hand, and occa- 

 sionally to the single phalanx of the thumb. The structure 

 of feathers is calculated in an eminent degree to combine 

 the qualities of lightness and of strength, which we else- 

 where rarely find united. The horny materials of which 

 the stem of the quill is made are tough, pliant, and elastic; 

 and, as we have already seen, are disposed in the most ad- 

 vantageous manner for resisting flexion by being formed 

 into a hollow cylinder. But the vane of the feather is still 

 more artificially constructed; being composed of a number 

 of flat threads, or filaments, so arranged as to oppose a much 

 greater resistance to a force striking perpendicularly against 

 their surface, than to one tvhich is directed laterally; that 

 is, in the plane of the stem. They derive this power of re- 

 sistance from their flattened shape, which allows them to 

 bend less easily in the direction of their flat surfaces than 

 in any other; in the same way that a slip of card cannot 

 easily be bent by a force acting in its own plane, though it 

 easily yields to one at right angles to it. Now it is exactly 

 in the direction in which they do not bend that the fila- 

 ments of the feather have to encounter the resistance and 

 impulse of the air. It is here that strength is wanted, and 

 it is here that strength has been bestowed. 



On examining the assemblage of these laminated filaments 

 still more minutely, we find that they appear to adhere to 

 one another. As we cannot perceive that they are united 

 by any glutinous matter, it is evident that their connexion 

 must be effected by some mechanism invisible to the unas- 

 sisted eye. By the aid of the microscope, the mystery is 

 unravelled, and we discover the presence of a number of 

 minute fibrils, arranged along the margin of the laminas, and 

 fitted to catch upon and clasp one another, whenever the la- 

 minae are brought w^ithin a certain distance. The fibrils of 

 a feather from the wing of a goose are represented magnified 



