614 



COMPARATIVE PHYSIOLOGY. 



Figs. 456 and 457 eliow that when the wings are elevated (e,^, g) the body falls (s): 

 and that when the wings are depressed (A, i, j) the body is elevated (r). Fig. 456 

 shows that the wings are elevated as short levers (e) until toward tbe termination 

 of the up-stroke, when they are gradually expanded (/, g) to prepare them for 

 making the down-stroke. Fig. 457 shows that the wings descend as long levers 

 (A) 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 mak- 

 ing the up-stroke. (Compare with Figs. 454 and 455.) By this means the air be- 

 neath the wings is vigorously seized during the down-stroke, while that above it 

 is avoided during 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 ob- 

 served, act as a parachute both during the up and down strokes. Fig. 457 shows 

 also the compound rotation of the wmg, how it rotates upon o, as a center, with 

 a radius m, b, n, and upon o, c, 6 as a center, with a radius k, I (Pettigrew). 



period of tlie cycle off the ground at tlie same time, owing to a 

 very energetic action of the foot acting as a fulcrum. 



Jumping implies the propulsion of the body by the impulse 

 given by both feet at the same moment. 



Hopping is the same act accomplished by the use of one 

 leg. 



Comparative. — The movements of quadrupeds are naturally 

 very complicated, but have now been well worked out by the 

 use of instantaneous photography. Even the bird's flight is no 

 longer a wholly unsolved problem, but is fairly well under- 

 stood. The movements of centipedes and and other many- 

 legged invertebrates are highly complicated, whUe their rapid 

 movements are to be accounted for by the multiplicity Of their 

 levers rather than the rapidity with which they are moved. 



