IV. VERTKBRATA: AVES 



are claws, but as a rule the fingers are feathered. On most places the 

 skin is soft and thin, since corium and stratum corneum are poorly devel- 

 oped. Periodic molts of the integument do not occur, since the horny 

 layer, as in mammals, undergoes a constant renewal. These peculiarities 

 of the skin are correlated with the appearance of the protecting plumage. 

 The feather, like the hair of mammals, is exclusively epithelial in 

 character, but is a much more complicated structure. The cornified 

 epithelium forms a firm axis, the scape, from which, right and left arise 

 branches, or barbs. The scape is solid as far as the barbs extend (rachis, 

 or shaft}, while below it is hollow (quill, or calamus}. The quill is 

 inserted deep in a follicle in the corium and is provided with muscles 

 for its movement. Its hollow in most fully developed feathers is empty 



FIG. 583. FIG. 584. 



FIG. 583. Feather tracts and apteria of pigeon, dorsal view (from Ludwig- 

 Leunis). 



FIG. 584. Regions and feathers of Falcc lanarius (from Schmarda). As, sec- 

 ondaries; Ba, belly; Br, breast; Bs, rump; D'-D", wing coverts; Di, gonys of bill; EF, 

 alula; F, culmen of bill; H, occiput; HS, primaries; K, throat; L, legs; N, neck: Sell, 

 crown; SF, parapterium; St, forehead, lower tail coverts; 5'c, rectrices; W, cheek; TJ'7/, 

 cere with nostril; Zk, toes. 



save for the 'pith,' a small amount of dried tissue. In young growing 

 feathers it is occupied by a richly vascular feather papilla, an outgrowth 

 of the corium for purposes of nourishment. The feather may therefore 

 be regarded as a cornified outgrowth from the skin which has arisen on a 

 papilla of the corium, a view which accords with its development and shows 

 its homology with the scales. In many birds (cassowaries) two well- 

 developed feathers arise from the same follicle a fact which explains the 



