50 



ANIMAL LOCOMOTION. 



portion of the trunk of the bird, the trunk being supported 

 for the time being by the left leg, which is extended, and in 

 contact with the ground. When the left leg is flexed, elevated, 

 and advanced, it in like manner rotates upon the trunk, sup- 

 ported in this instance by the extended right leg. The leg 

 which is on the ground for the time being supplies the neces- 

 sary lever, the ground the fulcrum. When the right leg is 

 flexed and elevated, it rotates upon the iliac portion of the 

 trunk in a forward direction, the right foot describing the arc of 

 a circle. When the right leg and foot are extended and fixed 

 on the ground, the trunk rotates upon the right foot in a for- 

 ward direction to form the arc qf a circle, which is the converse 

 of that formed by the right foot. If the arcs alternately supplied 

 by the right foot and trunk are placed in opposition, a more 

 or less perfect circle is produced, and thus it is that the loco- 

 motion of animals is approximated to the wheel in mechanics. 

 Similar remarks are to be made of the left foot and trunk. 

 The alternate rolling of the trunk on the extremities, and the 

 extremities on the trunk, utilizes or works up the inet tia of the 

 moving mass, and powerfully contributes to continuity and 

 steadiness of action in the moving parts. By advancing the head, 

 neck, and anterior parts of the body, the ostrich inaugurates 

 the rolling movement of the trunk, which is perpetuated by 

 the rolling movements of the legs. The trunk and legs of the 

 ostrich are active and passive by turns. The movements of 

 the trunk and limbs are definitely co-ordinated. But for this 

 reciprocation the action of the several parts implicated would 

 neither be so rapid, certain, nor continuous. The speed of 

 the ostrich exceeds that of every other land animal, a circum- 

 stance due to its long, powerful legs and great stride. It can 

 outstrip without difficulty the fleetest horses, and is only 

 captured by being simultaneously assailed from various points, 

 or run down by a succession of hunters on fresh steeds. 

 If the speed of the ostrich, which only measures six or eight 

 feet, is so transcending, what shall we say of the speed of the 

 extinct ^loyornis maximus and Dinornis giganteus, which are 

 supposed to have measured from sixteen to eighteen feet in 

 height] Incredible as it may appear, the ostrich, with its 

 feet reduced to a minimum as regards size, and peculiarly 



