VERTEBRATA: AVES. 



573 



white spongy substance, very like the pith of a plant. 3. The 

 shaft carries the lateral expansions or "webs" of the feather, 

 collectively constituting the "vane" 

 or " vexillum." Each web is com- 

 posed of a number of small bran- 

 ches, which form an open angle 

 with the shaft, and which are known 

 as the "barbs" (c\ The margins 

 of each barb are, in turn, furnished 

 with a seriesof still smaller branches, 

 which are known as the " barbules." 

 As a general rule, the extremities 

 of the barbules are hooked, so that 

 those springing from the one side 

 of each barb interlock with those 

 springing from the opposite side of 

 the next barb. In this way the 

 barbs are kept in apposition with 

 one another over a greater or less 

 portion of the entire web. More 

 or less of the barbs in the lower 

 portion of the feather are, however, 

 disunited, and not connected by 

 their barbules; and these consti- 

 tute what is known as the " down." 

 In the Ostriches, Emeus, and some 

 others, all the barbs of the fea- 

 thers are disconnected, giving to 

 the plumage of these birds its pecu- 

 liarly soft character. At the point 

 where the shaft joins the quill, and 

 on the under side of the former, 

 there is very generally found a small 

 feather, known as the "accessory 

 plume," or " after -shaft " (" hypo- 

 rachis "). This is usually much the 

 same in structure as the main fea- 

 ther, but considerably smaller. It 

 may, however, be as large as the ori- 

 ginal feather, or it may be reduced 

 to nothing more than a tuft of down. 



'ig. 320. Quill-feather (Stenopsis). 

 a Quill or barrel ; b Shaft ; i c 

 Webs, composed of the barbs, and 

 together forming the "vane." 



The feathers vary in different parts of the bird, and are generally divided 

 into those which cover the body "clothing feathers" and those which 

 occur in the wings and tail "quill-feathers." As regards the great quill- 

 feathers of the wings, the longest are those which arise from the bones of 



