CLASS AVES 



577 



rc7i 



drawn across the eyeball from the inner angle outward. Below 

 and behind each eye is an external auditory aperture (Fig. 471, B) 

 which leads to the tympanic cavity. 



The neck is long and flexible. At the posterior end of the 

 trunk is a projection which bears the tail feathers. The two 

 wings can be 

 folded close to 

 the body or ex- 

 tended as organs 

 of flight. The 

 hind limbs are 

 covered with 

 horny epidermal 

 scales, and their 

 digits are each 

 provided with a 

 horny claw. 



Feathers. — 

 Feathers are 

 peculiar to birds. 

 They arise, as 

 do the scales of 

 reptiles, from 

 dermal papillae 

 with a covering 

 of epidermis, 

 and become en- 

 veloped in a pit, 

 the feather fol- 

 licle. A typical feather (Fig. 472, A) consists of a stiff axial rod, 

 the scapus or stem ; the proximal portion is hollow, and semitrans- 

 parent, and is called the quill or calamus (cal) ; the distal portion 

 is called the vane, and that part of the stem passing through it 

 is the shaft or rachis (rch). The vane is composed of a series of 

 parallel barbs, and each barb bears a fringe of small processes, 

 2 p 



'■timb 



Fig. 472. — Feathers of the pigeon. A, proximal por- 

 tion of a contour feather. B, filoplume. C, nestling 

 down, cal, calamus; inf.umb, inferior umbilicus; rch, 

 rachis; sup.umb, superior umbilicus. (From Parker and 

 Haswell.) 



