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 



