THE PIGEON AS A TYPE OF BIRDS. 631 



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 produced is often enhanced by structural peculiarities 

 of texture and surface. In perfectly w r hite 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 hooklets. 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 effect 

 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 epi- 

 dermis 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 pro- 

 tective external sheath, and (b) an inner stratum Malpighii, which 

 becomes cornified and forms the whole feather. The process by which 

 this cylinder of cells becomes horny is remarkable ; in the upper part 

 ridges are formed, which separate from one another as a set of barbs, 

 the lower part remaining intact as the quill. When we pull the horny 

 sheath off a young feather, we disclose a set of barbs lying almost 



