598 BIRDS. 



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 

 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 papillse 

 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 

 parallel with one another, yet slightly divergent. The central pair 

 predominate, and fuse to form the rachis ; its 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 

 grow. 



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. The toes are clawed, and in young birds the same 

 is true of the thumb and first finger. Only in the embryos of the 

 hoatzin (Opisthocoi?ms) and of the ostriches (Struthio and Rhea) is the 

 third digit clawed. The beak is covered by a horny sheath, which is 

 annually moulted in the puffin. The dermis is very thin and vascular, 

 and is rich in tactile nerve-endings or Pacinian corpuscles, especially 

 abundant in the cere. The only skin gland — the preen gland — secretes 

 an oily fluid, with which the bird anoints its feathers. It is absent 

 in the ostrich, emu, cassowary, and kiwi, and in a few Carinate birds. 



Muscular system. — The largest breast muscle (pectoralis 



