246 STUDIES IN PHYSIOLOGY 



tinued through the feather to its tip as the shaft. The vane 

 or flat surface on either side of the shaft is composed in the 

 first place of barbs that branch out in a diagonal direction. 

 From the side of each barb toward the tip of the feather (that 

 is, distally) run off a row of little barbules that are supplied 

 with tiny hooks. Along the other (proximal) side of the 

 barb are plain barbules, and the hooks of the barbules just 

 below catch on to these barbules when the feather is smoothed 

 or " preened.'' The feathers on the wing overlap in such a 

 way that in the downward stroke the surface of the wing is 



FIG. 112. Portion of Feather, magnified. 

 sh = shaft. 6 = barb. 6 1 = barbule with hooks. 



continuous. When the wing is lifted for a second stroke, 

 the feathers separate and allow the air to pass between them 

 with little resistance. 



Not all the feathers of birds are as complicated as the 

 quill feathers just described. Ostrich plumes have no hooks 

 on the barbules, and hence the latter do not cling together. 

 Egret plumes consist of shaft and barbs only ; all barbules 

 are wanting. And finally, about the beaks of some birds are 

 simple hairlike structures, corresponding to the feather 

 shaft. Whatever its structure may be, however, a feather, 

 like a hair or a nail, is always a modification of the outer 

 layers of the skin. 



The Skin of Mammals. As scales are the distinguishing 

 outer covering of fishes and reptiles, and as birds are char- 



