THE PIGEON AS A TYPE OF BIRDS. 655 
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 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 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 in- 
dividual 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 papillz 
rarely occur all over the skin (¢.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 (4) 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 off the 
horny sheath of a young feather, we disclose a set of barbs lying almost 
- parallel with one another, yet slightly divergent. The central pair 
predominate, and fuse to form the rachis; their neighbours gradually 
become the lateral barbs. The external sheath falls off; the core 
of dermis is wholly nutritive, and disappears as the feather ceases to 
row. 
On the four toes and on the base of the legs there are horny epidermic 
scales, the presence of which reminds us of the affinities between Birds 
and Reptiles. Thetoesare always clawed. The thumb of Birds is often 
clawed; the second digit very rarely. Only in the embryo of the 
ostrich (Struthio) is the third digit clawed. The beak is covered by 
