TEGUMENTARY ORGANS 



377 



sheath. Both layers, however, have the same essential structure, 

 being composed of rounded or polygonal horny plates, whose endoplasts 

 are often distinctly retained even in the outer layers. The histological 

 metamorphosis of the feather will be described below, but the manner 

 in -which it acquires its ultimate complex general figure requires 

 particular attention. Every feather consists of the following parts: — 

 the quill continuous with the shaft, 

 or central axis of the feather, which 

 supports the horizontally expanded 

 vane, consisting of numerous long, 

 narrow, flattened lamina ; the barbs 

 or primary rays, pointed at their ex- 

 tremities and arranged with their 

 edges upwards and downwards more 

 or less perpendicularly on the shaft. 

 Arranged in a similar manner on the 

 barbs, are the barbules, which there- 

 fore are disposed more or less parallel 

 to the shaft ; from the sides of these 

 lastly, project short, toothed, curved, 

 interlocking processes. 



All parts of the feather are solid, 

 except the quill, which is hollow and 

 occupied only by a dry shrivelled 

 mass, the pith, in its upper part, while 

 below, during life, it receives the 

 pulp. Superiorly, on the under side, 

 where the quill joins the shaft, there 

 is a small aperture, which communi- 

 cates with the interior, with a short canal in the shaft, and with a 

 groove which runs along its under surface. 



It may be well to remember that the apex of a barbule resembles 

 in structure one of its own processes ; that of a barb, one of its 

 barbules ; that of the shaft, one of its barbs. 



The development of this complicated organ from its matrix or 

 pulp takes place very simply, by a sort of exaggeration of the com- 

 bination of hair development with that of the nails, which has 

 already been described as occurring in the spines of the porcupine. 



On the surface of the feather pulp a series of ridges are developed, 

 running pretty nearly parallel with one another from an antero- 

 posterior groove upon the upper surface, which marks the position of the 

 future shaft, to a line parallel with that groove upon the under surface or 



Fig. 306. — Sectional view of feather 

 in its sac : Fowl, e, barb ;/, pulp ; 

 ?•, vessel. 



