DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 329 



distal one (s) is directed upwards,""" and vice versa. The same holds true of the 

 axillary and distal curves occurring on the posterior margin of the wing 

 (t. u)A .... The anterior axillary and distal curves completely reverse them- 

 selves during the acts of extension and flexion, and so of the posterior axil- 

 lary and distal curves. This reversal of the curves is seen to most advantage 

 in the posterior margin of the wing, formed in the bird, by the primary, second- 

 ary, and tertiary feathers.;); When the wing is partially flexed the convexity 

 of the distal curve (occurring on the posterior margin of the wing) is directed 

 downwards (page 362, figure 41 a, &),§ that of the axillary curve upwards (a, c).\\ 

 When the wing is rather more than half extended the curves are obliterated, 

 the posterior margin of the wing becoming straight (page 362, figure 42 b, c).1F 

 It is at this stage of extension that the axillary and distal curves reverse. 

 When the wing is fully extended the convexity of the axillary curve is directed 

 downwards (page 362, figure 43 a, c),** that of the distal one upwards (a, b),ff 

 which is just the opposite of what happens in flexion. This antagonism in the 

 axillary and distal curves observed in the posterior margin of the wing of the bird 

 is referrible to changes induced in the anterior margin of the pinion, as the 

 subjoined paragraph will show." .... 



The Tip of the Bird's Wing describes an Ellipse. — " The movements of the 

 wrist are always the reverse of those occurring at the elbow joint. Thus, 

 during extension, the elbow and bones of the forearm are elevated, and describe 

 one side of an ellipse ; while the wrist and bones of the hand are depressed, and 

 describe the side of another and opposite ellipse. U These movements are reversed 

 during flexion, §§ so that when the elbow is raised and carried backwards, the 

 wrist is lowered and carried forwards, and vice versa." \\\\ .... 



The Wing capable of Change of Form in all its Parts. — " From this descrip- 

 tion it follows that when the different portions of the anterior margin are ele- 



" ;: ' Op. tit, Plate XV. fig. 73,/. t Op. tit, Plate XV. fig. 73, a, b, c. 



t Op. tit, Plate XV. figs. 73, 74, 75. § Op. tit, Plate XV. fig. 73, b. 



I] Op. tit, Plate XV. fig. 73, a, e. IT Op. tit., Plate XV. fig. 74, b, c. 



** Op. tit, Plate XV. fig. 75, c. ft QP- dt > Plate xv - fi g- 75 > «, b - 



%\ Op. tit., p. 249, Diagram 14. §§ Op. tit., p. 249, Diagram 15. 



I Similar movements occur in the body and tail of the fish in the act of swimming. " The double 

 curve or spiral into which the fish throws itself when swimming may be conveniently divided into an 

 upper or cephalic curve,* and a lower or caudal one.f "When the concavity of the caudal curve is biting 

 or laying hold of the water, and when the concave surface of the tail is being forced during extension 

 with great violence in the direction of the axis of motion, % where the concave surface is suddenly converted 

 into a convex one, the concavity of the cephalic curve, i.e., the concave surface of the upper half of the 

 fish, is being urged, with less vigour, in the direction of the same line from the opposite side of it. As the 

 caudal and cephalic curves are obliterated when the line in question is reached, there is, consequently, a 

 period (momentary it must be), between the effective and non-effective strokes, in which the body of the 

 fish is comparatively straight, and, consequently, in a position to advance almost without impediment." § 



* Op. cit., Diag. 2, d, p. 204. t Op. tit., Diag. 2, c, p. 204. J Op. tit, Ding. 2, a, I, p. 204 



§ Op. cit., p. 205. 



VOL. XXVI. PART II. 4 Q 



