BIRDS 



271 



Amidst the feathers above the base of the tail is the oil 

 gland which secretes an oily fluid that is distributed over 

 the feathers with the beak. 



Structure of a feather. — A feather consists of two main 

 parts, the quill and the vane (Fig. 184). The quill is a 

 hoUoWj horny stalk, that extends from the origin of the 

 feather to the vane. The vane is the flat, expanded portion 

 of the feather. It has a longitudinal axis, the rachis, 

 which is a continuation of the quiU but differs in being 

 soUd. Running obliquely to the right and left, on each 

 side from the rachis, are delicate, threadhke structures, 

 called the harhs, which are closely held together by other 

 delicate, threadlike structures running out from them, 

 termed barbules. The latter are interlocked with one an- 

 other, thus uniting the barbs and forming a continuous sheet. 

 The feathers grow from small conical projections of the 

 skin, called papillce. In this respect they differ from 

 hairs which grow from deep invaginations of the skin. 



Wings. — Each wing consists of three parts: the arm, 

 forearm, and hand (Fig. 185), which correspond with the 



_ It*'*-*'!'* 



Ulna 



Fig. 185. — Bones of sparrow's wing, 



::^rR'adius Handboms^ 



'era 



like parts of our own arm. The wings are concave on the 

 inside and fit snugly to the body, reaUy adding to the sym- 

 metry and beauty of the bird. When the wing is folded, 

 the arm, forearm, and hand form the letter Z, a position our 



