6oo BIRDS. 



there are little hair-like feathers (filoplumes) with only a few 

 terminal barbs. 



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

 into a lower hollow portion — the calamus or quill, and an uj^per 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. The 

 quill is fixed in a pit or follicle of the .skin, with which muscle fibres 

 are connected. At the base of the quill there is a little hole — the in- 

 ferior 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, e.xcept that para- 

 sites sometimes enter by it. Close to this region, however, in many 

 birds, a tuft or branch arises, which is called the aftershaft. In the 

 Emu and Cassowary, the aftershaft is so long that each feather seems 

 douljle. 



A feather grows from a papilla of skin, but the whole of the feather 

 is really formed from the cornification of the inner layer of the epidermis. 

 The papilla; rarely occur diffusely on the skin, but are usually disposed 

 along definite feather-tracts. Each papilla consists externally of epider- 

 mis and internally of dermis, and becomes surrounded by a depression 

 or moat, which deepens to form the feather-follicle or the sac in which 

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

 outer stratum corneum, which in the develo]:)ing feather forms merely a 

 protective external sheath, and (i^) 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 remains 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 flivergent. The central one predominates as 

 the rachis, and its neighbours gradually become the lateral barbs. The 

 external sheath falls off; the core of dermis is wholly nutritive, and dis- 

 appears as the feather ceases to grow. 



On the toes and on the base of the legs small epidermic 

 scales occur. The toes are clawed, and in some birds the 

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

 embryos of the hoatzin {Opistlwconius) and of the ostriches 

 {Striithio and Rhea) is the second finger 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 Paccinian corpuscles, which 

 are 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. 



