124 GENERAL ORNITHOLOGY part ii 



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 specialised, wonderfully beautiful and per- 

 fect kind of tegumentary outgrowths — 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, ex- 

 cepting birds, that progress through the air — the flying-fish, with its 

 enlarged pectoral fins ; the flying-reptile, Math 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. 

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



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 follicle is composed of several thin strata of nucleated 

 epithelial cells (cuticle cells) ; the inner is thicker, spong}^, 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 resolv- 

 ing 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 fine- 

 rayed pencil point. The process 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, which are termed ' feathers,' do not take the form of 

 mere plates developed upon the surface of the skin, but are evolved 

 within sacs from the surfaces of conical })apillc'e of the dermis. The 

 external suiface of the dermal 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 doi'sal 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 



