EXTERNAL CHARACTERS 397 



B. The Limbs. 



1. The wing. 



Identify the, several hones of the wing hy feeling them- 

 through the skin. Note also the varying positions of the sw- 

 faces of the several divisions of the wing, when it is extended 

 and folded respectively (p. 388). 



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

 of feathers attached to the phalanx of the first digit, 



2. The leg. 



The toes and the tarso-metatarsal region are, ex- 

 cept 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 features 

 of the bird. 



1. Structure of a feather. 



Pull out one of the la/rge 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 in 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, 

 there is a second aperture, the superior 

 umbilicus. 



ii. The rachis, or shaft, is the distal solid portion 

 of the stem. It is a tapering, flexible, elastic 

 rod, square in transverse section, and grooved 

 along its ventral surface. 



