PROGRESSION ON THE LAND. 



49 



The wings of the ostrich, although useless as flying organs, 

 form important auxiliaries in running. When the ostrich 

 careers along the plain, he spreads out his wings in such a 

 manner that they act as balancers, and so enable him to main- 

 tain his equilibrium (fig. 25). The wings, because of the angle of 

 inclination which their under surfaces make with the horizon, 

 and the great speed at which the ostrich travels, act like 

 kites, and so elevate and carry forward by a mechanical 

 adaptation a certain proportion of the mass of the bird 

 already in motion. The elevating and propelling power of 

 even diminutive inclined planes is very considerable, when 

 carried along at a high speed in a horizontal direction. The 

 wings, in addition to their elevating and propelling power, 

 contribute by their short, rapid, swinging movements, to con- 

 tinuity of motion in the legs. No bird with large wings can 

 run well. The albatross, for example, walks with difficulty, 

 and the same may be said of the vulture and eagle. What, 

 therefore, appears a defect in the ostrich, is a positive advan- 

 tage when its habits and mode of locomotion are taken into 

 account. 



Professional runners in many cases at matches reduce the 

 length of their anterior extremities by flexing their arms and 

 carrying them on a level with their chest (fig. 28, p. 62). It 

 would seem that in rapid running there is not time for the arms 

 to oscillate naturally, and that under these circumstances the 

 arms, if allowed to swing about, retard rather than increase 

 the spe^d. The centre of gravity is well forward in the 

 ostrich, and is regulated by the movements of the head and 

 neck, -and the obliquity of the body and legs. In running 

 the neck is stretched, the body inclined forward, and the legs 

 moved alternately and with great rapidity. When the right 

 leg is flexed and elevated, it swings forward pendulum- 

 fashion, and describes a curve whose convexity is directed 

 towards the right side. When the left leg is flexed and 

 elevated, it swings forward and describes a curve whose con- 

 vexity is directed towards the left side. The curves made by 

 the right and left legs form when united a waved line {vide 

 figs. 18, 19, and 20, pp. 37, 39, and 41). When the right 

 leg is flexed, elevated, and advanced, it rotates upon the iliac 



