Plumage and Skeleton of Birds. 5 5 



protoplasmic matter exuded by the surface of the 

 papillae is collected into these channels, it then hardens 

 and forms in the first place a hollow cylinder of horny 

 matter, with ridges fitting into the papillary grooves 

 and thin areas between. As growth takes place most 

 actively at the base of the papilla, this horny sheath 

 is pushed off the surface of the papilla, upon which it 

 shrinks still more, and the horny cylinder splits along 

 its thinnest side, whereupon the two lateral laminae 

 flatten out as the vanes, while the rib which corre- 

 sponded to the main groove of the papilla becomes 

 the rachis. The feathers of many birds are of brilliant 

 colours, usually brighter in the males than in the 

 females. 



31. Bones. The skeleton of birds is well adapted 

 for their aerial existence. The skull is early consoli- 

 dated, and articulates with the spine by one occipital 

 condyle. The ten or twelve parts of the lower jaw 

 are also early united into a single piece and the front 

 of the jaw is enclosed in a horny sheath ; it articulates 

 to the skull by means of a movable quadrate bone 

 which is kept in its place by two rods of bone, one 

 on the outside stretching from the base of the upper 

 beak (the jugal arch), the other on the inside stretch- 

 ing from the palate (the pterygoid arch). The upper 

 mandible, or beak, is also encased in a horny layer 

 at whose base are the nostrils, very often surrounded 

 by a thick leathery skin, which is called the cere. 



The neck is usually long and exceedingly flexible, 

 made up of from nine to twenty- three vertebrae ; its 

 length and that of the bill usually bears some propor- 

 tion to the length of the legs. The breast-bone bears 



