KINDS OF MOTION. 133 



latter bones. In bending the leg, this process is lifted out 

 of the depression, and removed to its posterior edge. By 

 this motion, the ligaments are necessarily more stretched 

 than during the extension of the leg, in which the process 

 remains in its socket. These ligaments, therefore, preserve 

 the leg extended in the manner of some springs, without 

 receiving any assistance from the muscles *." 



2. Walking,-*- This action is defined by CUVIER, to be a 

 motion on a fixed surface, in which the centre of gravity is 

 alternately moved by one part of the extremities, and sus- 

 tained by the other, the body never being at any time 

 completely suspended over the ground. It is produced by 

 the alternate flexion and extension of the limbs, aided by 

 the motions of the trunk, advancing the position of the 

 centre of gravity in the intended direction. 



In animals with many feet, as the myriapoda, walking 

 is performed by so uniform a motion, that the body may 

 be said to glide along the surface. The feet do not move 

 by pairs, but by divisions, containing from five to twenty, 

 and upwards. In the insects with six feet, the anterior and 

 posterior legs on one side, and the middle leg on the other, 

 are moved at the same time, so that the body is always 

 supported by two legs on the one side, and one leg on the 

 other. The hair on the rings of caterpillars, likewise 

 serve as feet in assisting progressive motion. In animals 

 with four feet, " each step is executed by two tegs only ; 

 one belonging to the fore-pair, and the other to the hind^ 

 pair ; but sometimes they are those of the same side, and 

 sometimes those of the opposite side/ 1 The latter is that 

 kind of motion in horses, which grooms term a pace. The 

 right fore-leg is advanced so as to sustain the body, which 

 is thrown upon it by the left hind-foot, and at the same 



* CUTIER'S Comparative Anatomy, Lect. vii. a, I. 



