PEOGRESSION IN OR THROUGH THE AIR. 187 



and necessitates its ascent. While the wing is ascending and 

 descending the oblique muscles cause it to rotate on its long 

 axis, the bipartite division of the wing at its root, the spiral 

 configuration of the joint, and the arrangement of the elastic 

 and other structures which connect the pinion with the body, 

 together with the resistance it experiences from the air, con- 

 ferring on it the various angles which characterize the down 

 and up strokes. The wing may therefore be said to be de- 

 pressed by the shortening of the antero-posterior set of 

 muscles, aided by the oblique muscles, and elevated by the 

 shortening of the vertical and oblique muscles, aided by the 

 elastic ligaments, and the reaction of the air. If we adopt 

 this view we have a perfect physiological explanation of the 

 phenomenon, as we have a complete circle or cycle of motion, 

 the antero-posterior set of muscles shortening when the 

 vertical set of muscles are elongating, and vice versa. This, I 

 may add, is in conformity with all other muscular arrange- 

 ments, where we have what are usually denominated exten- 

 sors and flexors, pronators and supinators, abductors and 

 adductors, etc., but which, as I have already explained (pp. 

 24 to 34), are simply the two halves of a circle of muscle and 

 of motion, an arrangement for securing diametrically opposite 

 movements in the travelling surfaces of all animals. 



Chabrier's account, which I subjoin, virtually supports this 

 hypothesis : — 



" It is generally through the intervention of the proper 

 motions of the dorsum, which are very considerable during 

 flight, that the wings or the elytra are moved equally and 

 simultaneously. Thus, when it is elevated, it carries with it 

 the internal side of the base of the wings with which it is 

 articulated, from which ensues the depression of the external 

 side of the wing ; and when it approaches the sternal portion 

 of the trunk, the contrary takes place. During the depres- 

 sion of the wings, the dorsum is curved from before back- 

 wards, or in such a manner that its anterior extremity is 

 brought nearer to its posterior, that its middle is elevated, 

 and its lateral portions removed further from each other. 

 .The reverse takes place in the elevation of the wings; the 

 anterior extremity of the dorsum being removed to a greater 



