82 GENERAL ORNITHOLOGY. 



§ 3. _ DEFINITIONS AND DESCRIPTIONS OF THE EXTERIOR PARTS OF BIRDS. 

 a. Of the Feathers, or Plumage. 



Feathers are possessed only by birds, and all birds possess them. Feathers are modifi(>d 

 scales; like scales, hair, horns, plates, sheaths, etc., tliey are outgrowths of the inte,i;iiiiieiit, or 

 skin covering the body, and therefore belong to the class of epidermic (Gr. eVi, epi, upon; 

 Se'pfia, derma, skin), or e.TOskeletal (Gr. e^, ex, out; o-kiXctov, skeleton, dried; in the sense ot 

 " outer skeleton") sti'uctures. The horny coverings of the beak and feet are of the same class, 

 but very differently developed. Besides being the most highly develoixid or coni]>lexly s])ecial- 

 ized, wonderfully beautiful and perfect kind of tegumentary outgrowth ; besides fulfilling in .i 

 singular manner the design of covering and protecting the body ; — feathers have their particular 

 tocomotory office ; that of accomplishing the act of Hying in a manner peculiar to birds. For 

 all vertebrates, excepting birds, that progress through the air — the flying fish { Exocajtufi) \y\th 

 its enlarged pectoral fins ; the flying reptile (Draco or Pterodactyl) with its skinny paracliute ; 

 the flying mammal (bat) with its great webbed fingers — accomplish aerial h.icomotion by means 

 of tegumentary expansions. Birds alone fly with t(!gumentary OM^(/rO!P</is, or appendages. All 

 a bird's feathers, of whatever kind, collectively constitute its ptilosis (Gr. TrriXoi', ptiloii, a 

 feather) or plumage (Lat. piluma, a plume or feather). 



Development of Feathers. — In a manner analogous to that of liair, a featlier grows in 

 a little pit or pouch formed by inversion of the dermal or true-.skin layer of the integument, 

 being formed in a closed follicle or shut sac consisting of an inner and outer cf>at separated by 

 a layer of fine granular substance. Tlie outer layer or " outer follicle" is composed of stn-eral 

 thin strata of nucleated epithelial cells (cuticle cells) ; the inner is thicker, spongy, and filled 

 with gelatinous fluid ; a little artery and vein i'urnish the blood circulation, very active during 

 the formation of feathers. Tlie inner is the true matrix or mould upon which the feather is 

 formed, evcdving from the blood-supply the gelatinous material, and res(dving this into cell- 

 nuclei; the granular layer is the formative material which becomes the featlier. The outer 

 grows a little beyond the cutaneous sac that holds it, and opens at the end; from this orifice 

 the future feather protrudes, sprouting as a little five-rayed pencil point. The jjrocess is thus 

 graphically illustrated by Huxley: "The integument of birds is always provided with horny 

 appendages, which result from the conversion into horn of the cells of the outer layer of the 

 epidermis. But the majority of these appendages, wliich are termed 'feathers,' do not talte the 

 form of mere plates developed upon the surface of the skin, but are evolved within sacs from 

 the surfaces of conical papilla? of the dermis. The external surfac(! of tlie dernuvl (lapilla, 

 whence a feather is to be developed, is provided upon its dorsal [upper] surface with a median 

 groove, which becomes shallower towards the apex of the papilla. Fnmi this median groove 

 lateral furrows proceed at an open angle, and passing round upiui the under surface of the 

 papilla, become shallower, until, in the middle line, opposite th(^ dcu-sal median groove, they 

 become obscdete. Minor grooves run at right angles to the lateral furi-ows. Ilenco the surface 

 of the papilla has the character of a kind of mould, and if it were repeatedly dijijied in such a 

 substance as a solution of gelatine, and withdrawn to cdol until its whole surface was covereil 

 with an even coat of that substance, it is clear that the gelatinons coat would be thickest at 

 the basal or anterior end of the median groove, at the median ends of the lateral furrows, 

 and at those ends of the minor grooves which open into them ; while it would be very thin 

 at the apices of the median and lateral grooves, and between the ends of the minor grooves. 

 If, therefore, the hollow cone of gelatine, rc^moved frcun its mould, were stretched from within; 

 or if its thinnest parts became weak by drying ; it would tend to give way, along the inferior 

 median line, opjHjsite the rod-Uko cast of the dorsal median groove and between the ends of 



