82 GENERAL OBNITHOLOGY. 



§ 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 modified 

 scales; like scales, hair, horns, plates, sheaths, etc., they are outgrowths of the integument, or 

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

 bipjxa, derma, skin), or exoskeletal (Gr. f$, ex, out; a-KeXcTov, skeleton, dried; in the sense of 

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

 but very differently developed. Besides being the most highly developed or complexly special- 

 ized, wondeifully beautiful and perfect kind of tegumentary t)utgro\inh ; besides fulfilling in a 

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

 locomotory office : that of accomplishing the act of flying in a manner peculiar to birds. For 

 all vertebrates, excepting birds, that progress through the air — the flying fish (Exocoetus) with 

 its enlarged pectoral fins ; the flying reptile {Draco or Pterodactyl) with its skinny parachute ; 

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

 of tegumentary expansions. Birds alone fly with tegumentary outgrowths, or appendages. All 

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

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



Development of Feathers. — In a manner analogous to that of hair, a feather 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 coat separated by 

 a layer of fine granular substance. The outer layer or " outer folhcle " is composed of several 

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

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

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

 formed, evolving from the blood-supply the gelatinous material, and resolving this into cell- 

 nuclei; the granular layer is the formative material which becomes the feather. 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 process is thus 

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

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

 epidermis. But the majority of these appendages, which are termed ' feathers,' do not take the 

 form of mere plates developed upon the surface of the skin, but are evolved within sacs fi-om 

 the surfaces of conical papillte of the dermis. The external surface of the dennal papilla, 

 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. From this median groove 

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

 papilla, become shallower, until, in the middle line, opposite the dorsal median groove, they 

 become obsolete. Minor grooves run at right angles to the lateral furrows. Hence the surface 

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

 substance as a solution of gelatine, and withdrawn to cool until its whole surface was covered 

 with an even coat of that substance, it is clear that the gelatinous 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, removed from 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, opposite the rod-like cast of the dorsal median groove and between the ends of 



