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 Cuvizr, 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 legs 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.” 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 

* Cuvier’s Comparative Anatomy, Lect. vii. a, 1. 
