270 



FIELD ZOOLOGY. 



stiff quills or primaries, the one lying lowest and under- 

 most being called the first primary. These are reinforced 

 by the secondaries, lying above the primaries, and gener- 

 ally of rounder and softer outline. The barbs of the pri- 

 maries are unequally developed on the two sides of the 

 central shaft; the barbs on one side being set at a higher 

 angle and often being of shorter length than the barbs 



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FIG. 106. Front limb, or wing, of a bird. (After Coues.) 



on the other side of the shaft. In the secondaries, the 

 barbs grow more nearly symmetrically. Complex muscles 

 connect with the epidermal sockets from which these 

 feathers grow; so that the bird, in flight, spreads the barbs 

 apart and turns them so as to " feather its oars" in the 

 aerial ocean which it inhabits. 



The upper arm is not free from the forearm, but a 

 fold of skin crosses the space between them and a strong 

 muscle supports this fold of skin between shoulder and 

 wrist, diminishing or increasing their approach to each 



