TEGUMENTARY ORGANS— SKIN STRUCTURES. 



457 



Structure. — An ordinary quill feather is a marvel 

 of lightness, strength, and elasticity. The quill is hollow 

 — a form which gives greatest strength for the same weight 



QP fh 



FIG. 313.— Section through the successive feathers of a bird's wing, showing 

 the mode of overlap : sh, shaft ; av, anterior, and pv, posterior vane. 



of material. The shaft may be regarded as a hollow 

 tube filled within with lightest bracing, with the horny 

 envelope thickest on the back, where the strain comes in 

 flight. On each side of the shaft is the vane. It will be 

 observed that the two parts of the vane are not equal, 

 the backward overlapped part,/ v, being much the broader. 

 The effect of this 

 is to close up the 

 feathers into a 

 solid plane in the 

 downward blow 

 of the wing, while 

 it opens the feath- 

 ers and lets the 

 air through in the sh r -\ 

 upstroke (Fig. 

 313). Again, the 

 vane consists of 

 barbs, the shafts of 

 which are broad 

 and therefore 

 strong in a verti- 

 cal direction, and 



on these barbs again are a vanelet on each side com- 

 posed of barbules, and finally the barbules are hooked 

 together by little elastic hooks (Fig. 314). In Fig. 315 



Fig. 314. — Portion of a shaft (sh) and vane on 

 one side, showing (6) the barbs and (6') the 

 barbules with their hooks. 



