734 PHYLUM CHORDATA : CLASS AVES — BIRDS 



The feathers most important in flight are the twenty- 

 three remiges of the wing, divided into eleven primaries 

 borne by the metacarpals and phalanges of the two fingers, 

 and twelve secondaries borne by the ulna. Twelve tail 

 feathers or rectrices serve as a brake, and help a little in 

 steering. A distinct tuft of feathers borne by the thumb 

 is called the bastard wing. Covering the bases of the large 

 feathers are the coverts — wing-coverts and tail-coverts — 

 which belong to the series of contour feathers which give 

 shape to the whole body. In the pigeon there are no true 

 down-feathers or plumules, but among the ordinary 

 contour feathers or pennae there are little hair-like feathers 

 (filoplumes) with only a few terminal barbs. In herons 

 and some other birds some of the down-feathers are 

 covered with dusty powder (powder-down) formed from 

 the brittle ends of the barbs. Apart from their use in 

 flight, the feathers, being bad conductors of heat, serve 

 to sustain the high temperature of the bird. There is 

 usually pigment in feathers, and the coloration thus pro- 

 duced is often enhanced by structural peculiarities of 

 texture and surface. In perfectly white feathers the 

 whiteness is due to gas-bubbles. 



Any one of the large feathers consists of an axis or scapus, divided 

 into a lower hollow portion — the calamus or quill, and an upper solid 

 portion — the rachis, which forms the axis of the vane. This vane con- 

 sists of parallel rows of lateral barbs, linked to one another by barbules, 

 which may be joined to one another by microscopic booklets. In the 

 running birds the barbs are free. The quill is fixed in a pit or follicle 

 of the skin, from which muscle fibres pass to the feather and efiect 

 individual movement. At the base of the quill there is a little hole — 

 the inferior umbilicus — through which a nutritive papilla of dermis is 

 continued into the growing feather. At the base of the vane there is 

 a little chink — the superior umbilicus — but this has no importance, 

 except that parasites sometimes enter by it. Close to this region, 

 however, in many birds, a tuft or branch arises, called the aftershaft. 

 In the Emu and Cassowary the aftershaft is so long that each feather 

 seems double. 



A feather begins as a papilla of skin, but the whole is formed from 

 the cornification of the inner layer of the epidermis. The papilla) 

 rarely occur all over the skin (e.g. penguin), but are usually disposed 

 along definite feather-tracts. Each papilla consists externally of 

 epidermis and internally of dermis, and becomes surrounded at the 

 foot by a moat, which deepens to form the feather-follicle in which the 

 base of the quill is sunk. The epidermis has two layers — (a) an outer 

 stratum corneum, which in the developing feather forms merely a 



