Sauropsida: Class Aves 



505 



flight. This illustrates well the extraordinary importance of the feather 

 in the life of the bird, justifying our regarding the feather as the most 

 significant and distinctive of the many avian anatomic peculiarities. 

 It now remains to examine somewhat more fully these several related 

 structures, and, in doing so, they will be considered in the serial order 

 of the preceding paragraph. 



Feathers 



The feather is the characteristic integumentary structure of a bird, 

 but on the legs, below the feathers, are horny scales of reptilian type. 



A feather is an elaboration of the epidermal stratum corneum. The 

 fully developed feather is constituted entirely of quite lifeless horny 

 epidermal tissue (excepting some dead vestiges of embryonic dermal 

 material). A feather of the most complex type, such as a wing-feather 

 or one of the larger feathers of the body, is deeply but very obliquely 

 inserted in the skin. The inserted part is a hollow cylindric quill 

 (calamus), at whose proximal end is an aperture, the umbilicus 

 (Fig. 395). There may be a second umbilicus at the distal region of the 

 quill. The cavity of the quill contains a dry, spongy pith which is the 

 dead remnant of living vascular dermal tissue present there while the 

 feather is developing. External to the skin, the axis of the feather is the 

 solid shaft (rachis), from whose opposite sides extend barbs arranged 

 in bilateral symmetry with reference to the axis of the feather. Each 

 barb bears bilaterally arranged barbules. The barbules on the distal 

 side of one barb overlap the barbules on the proximal side of the 



Fin. 395. Diagram of base of contour feather. (A) Portion of barb showing 

 the barbules and hooks, (a) Aftershaft; (b) barbs; (bl) barbules; (//) hooks on ends 

 of barbules; (lu) lower umbilicus; (q) quill; (s) shaft; (u) umbilicus; (V) vane. 

 (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," Philadelphia, The 

 Blakiston Company.) 



