AVES. 



239 



attached to the shoulder, the longest and most 

 powerful bone of the wing, are tertiaries. The fea- 

 thers that overlap the bases of these are called 

 coverts, of which there are three rows, and still 

 nearer to the body are feathers called scapulars. 

 The tail consists of strong feathers resembling the 

 quills of the wing, but having both sides even ; 

 they are usually, not always, twelve in number. 

 Above them is a range of tail-coverts. All these, 

 as well as the general scale-like feathers of the body, 

 are beautifully and smoothly laid one upon another, 

 an arrangement that materially facilitates the action 

 of flying ; for while on the one hand they offer no 

 resistance to motion forward, but lie closer and 

 closer as the pressure of the air is increased, on 

 the contrary, the impulse backward produced by 

 the bringing forward of the wings after each stroke, 

 is resisted by the free edges of the feathers rising 

 and catching the air ; a resistance which increases 

 with the pressure. 



Each feather consists of two parts ; a light but 

 firm shaft formed of a pithy substance, hollowed at 

 the lower end into a horny tube, containing the 

 blood-vessels by which it is sustained ; and the vane, 

 a double series of parallel thin plates, one on each 

 side the shaft, set at an angle to it, which are them- 

 selves furnished at their edges with a similar though 

 smaller series. In all feathers which are destined 

 to strike the air, these branchlets are hooked into 

 one another, so as to present a continuous surface of 

 astonishing firmness. The brilliant hues, and often 



