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 riuill is a 

 hollow, horny stalk, that extends from the ojigin of the 

 feather to tlie vane. The vane is the flat, expantled portion 

 of the feather. It has a. longitu(Unal axis, the rachis, 

 which is a, continuation of the iiuill but differs in being 

 solid. Running obliquely to the right and left, on eacli 

 side from the rachis, are delicate, threadhke structures, 

 called the harhs^ which are closelj' held together lay other 

 delicate, threadlike structures ru.nning out from them, 

 termed harhules. The latter are interlocked Mith one an- 

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

 The feathers gi-ow from small conical projections of the 

 skin, calle^l papillce. In this I'esfject they (liffer 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 



Humerus 



Ulna 



• Carpal bones^^r^^^^^ 

 '/jj^ '^' Fingers 



Hand bones 



I'^iG. 1S5. — Bones of .sparrow's wing. 



like parts of our owyl arm. The A\'ings are concave on the 

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

 metry and beauty of the bird. When the ^^dng is folded, 

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



