40 MATERIALS FOR A MEMOIR ON 



One, two, or three limbs may be on the ground at the same time. 

 The rates at which the succession of the foot-falls occur, in their 

 turn, depend not only upon the rate of speed at which the animal 

 is moving, but on the gait as well. (See p. 59 and infra.) 



The Positions of Limbs. 



In studying the motions of the limb of a vertebrate the position 

 which answers to that taken by the salamander, when at rest, is 

 assumed to be the best adapted for comparison. In this position 

 the limb is horizontal to the plane of the longitudinal axis of the 

 body. The ventre of the body and the ventral surface of the limb 

 are on the same plane nearly. The limb of a reptile varies scarcely 

 at all from that jusit named. When a terrestrial animal is erect 

 the limb instead of being on the same plane with that of the body 

 is moved a quarter of a circle downward. In the bird the posterior 

 extremity when at rest is in the same position as the terrestrial, 

 but the anterior extremity, in marked contrast to it, is flexed. 

 When extended the extremity is thrown upward to a position as 

 far removed from the horizontal position of the salamander in one 

 direction as is that of the terrestrial quadruped in the other. 



In the movement of all limbs the directions in the main are 

 forward and backward. The forward movement will be described 

 SiS forward strain, and the backward movement as backward strain. 

 Both the movements are oblique, but between them is a position 

 which is straight. In the terrestrial animal this position may be 

 said to answer to a line in the anterior extremity which lies imme- 

 diately in advance of the Mithers and in the posterior extremity 

 to the centre of the acetabulum. 



THE ACTION OF THE LIMB WHEN ON THE 

 GROUND. 



Kinds of Work done by the Limbs. 

 The kinds of work done by the limbs are two in number, — 

 viz., that done by the fore limbs and that done by the hind limbs. 

 The hind limbs are more powerful than the fore limbs, and in 

 some animals, as the kangaroo and the jumping-mouse, are the 

 main effectives. No terrestrial animal depends for support upon 

 the fore limbs. When all the limbs are equal, or nearly equal, 

 in length, the preponderance is still in favor of the hind limbs. 



