CHARACTERS OF AVES. 



425 



ther, and consists of a hollow horny cylinder. 2. The " shaft," 

 (b) which is simply a continuation of the quill, and which forms 

 the central axis of the feather. 

 The inferior surface of the shaft 

 always exhibits a strong longitu- 

 dinal groove, and it is composed 

 of a horny external sheath, con- 

 taining a white spongy substance, 

 very like the pith of a plant. 3. 

 The shaft carries the lateral ex- 

 pansions or "webs" of the fea- 

 ther, collectively constituting the 

 "vane." Each web is composed 

 of a number of small branches, 

 which form an open angle with 

 the shaft, and which are known 

 as the "barbs" (c). The margins 

 of each barb are, in turn, furnish- 

 ed with a series of 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, how- 

 ever, disunited, and not connected 

 by their barbules ; and these con- 

 stitute what is known as the 

 "down." In the Ostriches, 

 Emeus, and some others, all the 

 barbs of the feathers are discon- 

 nected, giving to the plumage of 

 these birds its peculiarly soft 

 character. At the point where 

 the shaft joins the quill there is 



very generally found a small feather, known as the " accessory 

 plume," or "plumule." This is usually much the same in 

 structure as the main feather, but considerably smaller. It 



Fig. 166. Quill-feather (Stenopsis). 

 a Quill or barrel ; b Shaft ; c c 

 Webs, composed of the barbs, and 

 together forming the "vane." 



