284 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 32 



this braking effect appears, at first, simply to be friction between 

 specially-shaped roughened areas on the feathers, which come into 

 contact at the critical moment. The difference between the texture 

 of the upper surface of a feather in one of these areas, and elsewhere, 

 can quite easily be seen with the naked eye. Figure 18 shows the 

 extent of one of them in a typical emarginated feather. 



Figure 18. — Upper surface of a slut-formiug feather in a griflfon 

 vulture's wing. The dotted line shows the limits of the friction 

 area 



But examination of the surface with a microscope indicates that 

 the roughness is more than a friction surface ; it shows that the effect 

 is brought about by thousands of tiny hooks which stand out above 

 the main surface, and engage with the ribbed underside of the broad 

 part of the overlapping feather. These hooks are really an extension 

 of the normal mechanism that holds the barbs of a feather together. 



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Figure 19. — Construction of the upper surface of a slot- 

 forming feather from a griffon vulture's wing. This sec- 

 tion is outside the friction area. Only a few of the 

 branches have been drawn in 



From the shaft of any feather the barbs branch off at an angle 

 inclined toward the tip. From them the barbules (Fig. 19) spring. 

 Those that are on the side of the barbs nearest the root of the feather 

 are simply spines that lie in serried ranks, springing at a fine angle 

 from the barb, but those that are on the side nearer the tip are much 

 more complex in structure. Figure 20 shows a typical example of 



