THE EXTBENAL STEUOTUEB OE BIEDS. I37 



investment. It is beautifully adapted to harmonize with the 

 rest of their organization, being extremely light, warm, and non- 

 conducting. It thus serves most effectively to maintain that 

 high temperature which distinguishes, their class. 



Beasts are provided with hairs, but feathers are much more 

 complicated and elaborate organs. They are, in fact; the most 

 complicated of all the appendages of the skia which any 

 animals possess. 



Whatever may be their modifications of size, colour, or tex- 

 ture, they are all formed on one common plan. Each feather 

 consists of a firm central axis, the base of which is the " quill," 

 and the part ahove this the " rachis " or " seapus " or " shaft," 

 to which the web, vendllum or pogonium, is attached on either 

 side. The " webs " of both sides of the " rachis," taken to- 

 gether, constitute the vane. The quill is implanted in the skin 

 and has two apertures, one at either end. Into the lower 

 — the umlilicus inferior — the soft vascular " pulp " of the 

 feather penetrates. The other aperture is called the umbilicus 

 svperior. The " vane " consists, as before said, of the flat- 

 tened expanded parts on both sides of the central axis, and each 

 lateral portion of it (the fore or outer web, fig. 141, F, or the 

 hiad or inner web, H.V.) is made up of a number of elongated 

 closely arranged laminse called " barbs ;" while from the 

 margins of each barb much smaller processes project, called 

 " barbules " or " radii," and the sides of the barbules may 

 also be furnished with still smaller processes or " barbulets," or 

 barbicels, or hamuli, or hooMets. Not unf requently a second shaft, 

 called an "■ aftershaft," H.E., springs from the summit of the quUl, 

 and this is generally a miniature representation of the normal 

 " shaft " with its " vane." The large feathers of the wing 

 and tail never have an aftershaft. They present a striking 

 combination of the two generally opposite characters — strength 

 and lightness — in a very high degree, as the barbules in- 

 terlock and keep the whole structure remarkably firm and 

 coherent. This kind of feather is called pennaoeous. Certain 

 feathers ia which these parts are separate, and which also have 

 long barbules, are very much looser in structure, and are called 

 "plumes," and their structure is termed plumulaceous — such as 

 those of the Ostrich. 



Most Birds are provided with more or less " down." Down 

 consists of very soft feathers, which may or may not have 

 an aftershaft, and may have no rachis at all, the soft barbs 



