DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 383 



inclined surfaces, two directed upwards and outwards, viz., e /and a b of fig. 9, 

 Plate XII., and two directed upwards and inwards, viz., c d and g h of fig. 9, 

 Plate XII. Those surfaces when the wing is moving are ever varying, and cause 

 the different portions of the pinion to act like so many kites. Thus, during 

 extension, the two portions of the wing marked a b and ef (fig. 9, Plate XII.,) 

 fly outwards and upwards, the two portions marked c d and g h (fig. 9, Plate 

 XII.,) flying inwards and upwards during flexion. As the two portions of the 

 wing, marked a b and ef, draw a current after them during extension, on which 

 the two portions marked c d and g h operate during flexion, it follows that one 

 part of the wing, whatever its position in space, makes a current on which another 

 portion inevitably acts. This result is facilitated by the manner in which the 

 primary and secondary feathers rotate upon their long axes in flexion and 

 extension, and also by the ascent and descent of the wing, inasmuch as 

 flexion always occurs towards the end of the down stroke, and extension 

 towards the end of the up stroke. The wing, I may add, as a rule produces 

 a current during the up stroke on which it operates during the down 

 stroke and vice versa. The inclined surfaces represented at fig. 9, Plate 

 XII., are reproduced in the partly extended wing at fig. 10, Plate XII., and a 

 comparison of the arrows marked by the same letters in the two figures will 

 show that the angles of inclination formed by the surfaces in question are some- 

 what changed. The wing when fully extended is seen at fig. 11, Plate XII. 

 Complete extension is followed by the obliteration of the inclined surfaces 

 indicated by the arrows a b, e f, c d, g h of figs. 9 and 10, Plate XII. The 

 obliteration of the inclined surfaces a b, ef, c d, g h of figs. 9 and 10, Plate XII., 

 is followed by the production of other inclined surfaces, these being occasioned 

 by the rotation of the wing upon its anterior margin (long axis) towards 

 the termination of extension. The angles of inclination formed by the under 

 surface of the wing in the extended condition are greatest towards the root and 

 least towards the tip of the wing, as shown at q p o of fig. 16, Plate XIII. 

 When the gannet's wing is extended and flexed by the aid of the hand, as repre- 

 sented at figs. 16 and 17, Plate XIII. , it shows the screwing and unscrewing 

 action of the pinion to perfection ; the dorsal and ventral surfaces of the wing- 

 oscillating on either side of a given line — the dorsal surface appearing above the 

 line in flexion (figs. 17, Plate XIII.,) and the ventral surface under the line in 

 extension (fig. 16, Plate XIII.) The upward and downward screwing of the 

 wing in flexion and extension is also shown at fig. 8, Plate XII. — the wing to 

 the right of the observer being flexed, and having its anterior margin (d ef) 

 directed slightly downwards (vide arrow), the wing to the left being extended, 

 and having its anterior margin (d' e'f) directed decidedly upwards (vide arrow.) 

 The Angles of Inclination which the Under Surface of the Gannet's Wing 

 makes with the Horizon in Extension and Flexion vary. — When the wing of the 



VOL. XXVI. PART II. 5 G 



