AVES 



259 



The skin of a bird is thin and the body is not uniformly 

 covered with feathers (Fig. 213). There are many bare spots, 

 as may be. readily seen by turning back the feathers on the neck 

 or breast. The bare spaces, apteria, are concealed by the over- 

 lapping feathers. Note where the feathers grow. These spaces 

 are called pterylw, from two Greek words meaning "feather 

 forest." 



Feathers are homologous with scales, as "the feather may be 

 regarded as a cornified outgrowth from the skin, which has arisen 



Fig. 



209. — Ichthyor'nis victor, X J. 

 (Restored by Marsh.) 



Fig. 210. — Hesperor'nis regal'is, XyV' 

 (Restored by Marsh.) 



on a papilla of the derma. "^ A large wing feather, for example, 

 is made up of the central stalk and the expanded part, or vane. 

 The hollow portion of the stalk nearest the body is the quill, 

 and the remaining part, the rachis or shaft. The vane is com- 

 posed of side branches or barbs, the barbs of side branchlets or 

 barbules, which are provided with hooks. The hooks of one 

 barbule interlock with those of the next and thus hold the parts 

 of the vane together. In down feathers the hooks are lacking. 



' Hertwig. 



