660 ANIMAL PHYSIOLOGY. 
Fics. 498 and 499 show that when the wings are elevated (e. f, 9), the body falls (s); and that 
when the wings are depressed (h, z, j), the body is elevated (r). Fig. 498 shows that the 
wings are elevated as short levers (e) until toward the termination of the up-stroke, when 
they are gradually saver (Ff, g) to prepare them for making the down-stroke. Fig. 499 
shows that the wings descend as long levers (h) until toward the termination of the down- 
stroke, when they are gradually folded or flexed (i, j) to rob them of their momentum and 
prepare them for making the up-stroke. wares with Figs. 496 and 497. By this means 
the air beneath the wings is vigorously seized during the down-stroke, while that above it 
is avoided ues the up-stroke. The concavo-convex form of the wings and the forward 
travel of the body contributes to this result. The wings, it will be observed, act as a para- 
chute both during the up and down strokes. Fig. 499 shows also the compound rotation 
of the wing, how it rotates upon a, as a center, with a radius m, b, n, and upon a, ¢, b as 
a center, with a radius k, l (Pettigrew). 
movements are to be accounted for by the multiplicity of their 
levers rather than the rapidity with which they are moved. 
e cA 
c tts 
ea a wy 
es: 
Fra. 500.—Chillingham bull (Bos Scoticus). Shows powerful heavy body, and the small 
extremities adapted for land transit. Also the figure-of-8 movements made by the feet 
and limbs in walking and running. uw, ¢, curves made by right and left anterior extremi- 
ties ; r, s, curves made by right and left posterior extremities. The right fore and the 
left hind foot move together to form the waved line (s, u); the left fore and the right hind 
foot move together to form the waved line (r, t). The curves formed by the anterior (#, u) 
and posterior (r, s) extremities form ellipses (Pettigrew). 
