380 DP PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



from before backwards, and from above downwards, the spiral currents 

 from the two wings impinging upon the sides of the bird, which is wedge 

 shaped, in such a manner as to force it upwards and forwards. The currents 

 also cross and neutralise each other below the body of the bird, and thus supply 

 additional buoyancy and propelling power. The arch made by the wing of the 

 gull, when fully extended and ready to give the down stroke, is seen at 3, 3' of 

 figure 18, Plate XIV. ; that made at the middle of the down stroke at figure 

 15, Plate XIII. ; and that made at the end of the down stroke at figure 13, 

 Plate XIII. The arch made by the wing of the gannet in extreme extension is 

 shown at figure 16, Plate XIII. 



The Primary, Secondary, and Tertiary Feathers Imbricate or Overlap. — 

 Another point of interest in the bird's wing is the manner in which the various 

 feathers (primary, secondary, and tertiary) overlap (fig. 20, Plate XIV.), and 

 the varying degrees of strength which they exhibit. Proceeding from the tip of 

 the wing towards the root we find as a rule that the first three primary feathers 

 are longer and stronger and overlap more than the second three — the second 

 three being longer, stronger, and overlapping more than the third three. 

 These points are well seen in the acuminate scythe-like wing, of which that 

 of the gull (fig. 15, Plate XIII.) and gannet (fig. 16, Plate XIII.) are good 

 examples.* Similar remarks may be made of the secondary and tertiary feathers, 

 as a reference to p q of fig. 16, Plate XIII. , will show. Another not less inte- 

 resting feature is the varying position of the vanes of the primary, secondary, and 

 tertiary feathers. Thus, in the first primary the vane {efoi fig. 49, page 379), 

 is placed quite on the anterior margin (c d) the posterior margin (a b) being three 

 or four times broader than the anterior one to admit of overlapping. The vane of 

 the feather occupies a more and more central position as we proceed from the tip 

 in the direction of the root of the wing, as shown at h ijkl of fig. 45, page 376, and 

 also at 1, 2, 3, 4, 5, 6, 7, 8, 9, j k I m n, &c, of fig. 20, Plate XIV. The first 

 primary, as will be seen from this account, is eccentric in its nature. It is more 

 eccentric than the second — the second being more eccentric than the third, 

 and so of all the primary and secondary feathers, until the stem of the feather 

 is found to occupy its centre. The posterior margin of the first primary, as a 

 consequence, rotates more than that of the second — the second than the third, 

 and so of the others — the valvular action of the wing being most marked at the 

 tip of the pinion, and gradually diminishing in the direction of the root. The 

 rowing feathers are necessarily eccentric. If the axis of each feather was not 

 placed nearer the anterior than the posterior margin the anterior margin would 

 rise as much as the posterior margin is depressed. This, however, is prevented 



* In some cases, as for instance in the more rounded form of wing shown in fig. 20, Plate 

 XIV., the 4th, 5th, and 6th primaries are longer and stronger, and overlap more than the 1st, 2d, and 

 3d. 



