rilOGRESSION IN OR THROUGtl THE AIR. 



175 



pinions. It is corrected in some by curving the body, in 

 others by bending or straightening the limbs and antennae, 

 but principally in all by the judicious play of the wings 

 themselves. 



The wing of the bat and bird, like that of the insect, is 

 concavo-convex, and more or less twisted upon itself (figs. 

 94, 95, 96, and 97). 



Fig. 95. 



Fig. 94.— Right wing of the Bat (PhyllorJiina gracilis), dorsal surface, d ef, 

 Anterior or thick margin of the wing, supported by the bones of the arm, 

 forearm, and hand (first and second phalanges) ; c ah, posterior or thin 

 margin, supported by the remaining phalanges, by the side of the body, and 

 by the foot. —Original. 



Fig. 95.— Right wing of the Bat (Phyllorhiaa gracilis), as seen from behind and 

 from beneath. When so regarded, the anterior or thick margin (df) of the 

 wing displays different curves from those seen on the posterior or thin mar- 

 gin (b c) ; the anterior and posterior margins being arranged in different 

 planes, as in the blade of a screw propeller. — Original. 



The twisting is in a great measure owing to the manner in 

 which the bones of the wing are twisted upon themselves, and 

 the spiral nature of their articular surfaces ; the long axes of 

 the joints always intersecting each other at nearly right angles. 

 As a result of this disposition of the articular surfaces, the 

 wing is shot out or extended, and retracted or flexed in a 

 variable plane, the bones of the wing rotating in the direction 

 of their length during either movement. This secondary 

 action, or the revolving of the component bones upon their 

 own axes, is of the greatest importance in the movements of 

 the wing, as it communicates to the hand and forearm, and 



