Sauropsida: Class Aves 



EPIDERMIS: 

 / Stratum 

 ' ,germinativum 

 J /Stratum corneum 



Rachis 



Depressor muscle / Dermis \ Inferior umbilicus 



Erector muscle Papilla 



Fig. 397. Feather follicles: diagrammatic section perpendicular to surface of skin. 



feather in relation to the surface of the body may be changed. By use 

 of this mechanism, the bird is able to erect ("fluff up") the plumage 

 when sleeping or in response to cold. 



Feathers exist in great variety of form. The larger feathers which 

 are effective in giving the characteristic form to the body of the bird 

 are called "contour feathers" (or plumes). These are usually of the 

 complex structure which has just been described. Much simpler are 

 down-feathers (plumules), which are comparatively small and con- 

 sist of a tuft of filaments (barbs) springing from the distal end of the 

 small quill, but without a rachis (Fig. 399C). Barbules, if present, are 

 not interlocked. Most simple are "hair-feathers" (filoplumes), 

 consisting of a hairlike rachis without barbs, or having merely a small 

 vestige of a vane at the tip of the shaft. Feathers intermediate between 

 one and another of these three types exist in great variety. "Powder 

 down-feathers" are plumules which lie beneath the contour feathers 

 on the ventral surface of the body of some birds, especially carnivorous 

 birds. They have the curious property of disintegrating to form a fine, 

 soft, powdery substance like a layer of "talcum powder" covering the 

 skin and perhaps serving to protect the ventral skin from becoming 

 soiled by contact with the not always immaculate interior of the nest. 



The feathers which are first developed on the young bird are 

 plumules ("nestling down-feathers"). These are succeeded by con- 

 tour feathers which, however, are usually restricted to certain well- 



