434 THE BIRDS xvi. 3 



surface of the papilla is ridged all round and the result is to produce a 

 number of fine threads or barbs of keratin, covering the body with 

 a coat of fluff, which acts as a heat insulator by preventing air circula- 

 tion. 



Feathers, like other epidermal structures, are moulted, either at a 

 certain stage in the life-cycle or seasonally, a new generation being 



d flflb 



v. r / 



Fig. 250. The structure of a feather. 

 I. Whole feather showing calamus (quill), C; rhachis (shaft), R; and vane, V. On the right 

 side a small area is shown as it appears under a lens; B, barb; Bs, barbule. II. A section cut 

 at right angles to two barbs in the plane of the barbules of the anterior series (Bsa). Note 

 how the hamuli, H, of the anterior barbs interlock with ridges (r) on the posterior barbules 

 {Bsp). III. Shows one anterior and two posterior barbules isolated. (After Pycraft, A History 

 of Birds, Methuen & Co., Ltd.) 



produced from the old papillae. Most birds moult after the breeding- 

 season, some a second time during the year. The down-feathers of the 

 nestling are partly replaced by contour feathers; the follicle, instead 

 of producing equal barbs, now forms two large ones at one side, which 

 together become the central axis (rhachis), carrying a series of further 

 barbs that spread at right angles to it to form the vane (vexillum). Each 

 feather (Fig. 250) thus consists of a central rhachis, forming the hollow 

 calamus or quill below and carrying the barbs, which make the vane. 

 The calamus opens at the base by the inferior umbilicus, the entrance 

 of the mesodermal papilla, and at the beginning of the vane there is a 

 second hole, the superior umbilicus. At this point there is often a loose 



