142 



CHARACTERS OF VERTEBRATE ANIMALS 



covered by overlapping scales. The characteristic parts of the 

 skeleton, however, are the feathers (fig. 100), structures which are 

 possessed by all birds, but by no other animals. They do not cover 

 the entire surface of the body, but are limited to certain feather 

 tracts {pterylcz}, between which are bare patches (apteria). 

 Feathers, like hairs, spring from pits in the skin, and are renewed 

 from time to time, during a moulting season. They are of three 

 kinds: i. small downy filoplumes, consisting of a stalk with a tuft 



A. 



FAR BARBULES 



NEAR BARBULE 



VANE 



INFERIOR 

 UMBILICUS 



Fig. TOO. Structure of a Feather (there is no after-shaft in the Pigeon) 

 A, Base of quill-feather; B, barbs and barbules (much enlarged); c and D, separate barbules (still further enlarged). 



of filaments at the end; 2. contoiir feathers, which cover most 

 of the body, and, as the name indicates, determine its shape ; and 

 3. large quill-feathers used in flight, and attached to the wings and 

 tail. The last two kinds closely resemble one another in structure, 

 and it may suffice to describe one of the quills, which are the most 

 specialized and characteristic of feathers. Both light and strong, 

 with a large surface for opposing to the air, they are exquisitely 

 adapted to their purpose, even in the minutest details of their 

 structure. We can distinguish between a central shaft, consisting 

 of a hollow quill continued into a solid rachis, and oblique fila- 

 ments or barbs projecting from each side of the rachis and 

 constituting with it the vane. Each barb is provided with an 

 oblique set of barbules on its far or distal side, facing the end of 

 the feather, and a similar set on its near or proximal side, facing 

 towards the base of the feather. The barbules of the distal row 

 are provided with hooklets, which catch on to ridges possessed 

 by the row of proximal barbules immediately in front of them. 



