THE FEATHERS. 431 



B. The Limbs. 



1. The wing. 



The surface of the wing is increased by the two 

 alar membranes at the armpit and shoulder. Its 

 inferior concavity grips the air, and the chief work 

 in flight is done by the front edge of the wing. 

 Hence the specially long and strong flight-feathers. 



Identify the several bones of the wing by feeling them through 

 the skin. Note also the varying positions of the surfaces of 

 the several divisions of the wing, when it is extended and folded 

 respectively (p. 422). 



The ala spuria, or bastard wing, is a small tuft 

 of feathers attached to the phalanx of the preaxial 

 digit. 



2. The leg. 



The toes and the tarso-metatarsal region are, 

 except in some domesticated breeds, devoid of 

 feathers and covered with large horny epidermal 

 scales. The ends of the toes are provided with 

 horny claws. 



C. The Feathers. 



The possession of feathers, which are specially modified 

 epidermal structures, is one of the most characteristic fea- 

 tures of the bird. 

 1. Structure of a feather. 

 Pull out one of the large feathers from the wing or tail ; 

 and note in it the following points : 



a. The scapus, or stem, is a stiff axial rod, running 

 the whole length of the feather. It consists of 

 two dissimilar portions : 

 i. The calamus, or quill, is the tubular semi- 

 transparent proximal portion, the base of 

 which is inserted into the skin. At the 

 proximal end of the quill there is a small 

 aperture, the inferior umbilicus ; and at its 

 distal end between the quill and the vane, 



