452 
those of the posterior; this tends to throw 
the centre of gravity a little in advance of the 
middle of the quadrilateral figure described by 
the four legs of the quadruped on the plane 
of position. 
Although the bones of the posterior extre- 
mities are inclined to each other at a greater 
obliquity than those of the anterior, they have 
the calcaneum projecting considerably beyond 
the axis of the palowhondibial articulation, so 
that, acting with the advantage of a powerful le- 
ver, the waste of muscular force is diminished,and 
the disadvantage arising from their obliquity is 
partially compensated. The time and order in 
which the legs of quadrupeds succeed each 
other in motion determine the deno- 
minated the walk, the trot, the gallop, the 
amble, and the leap, or bound. 
In order to illustrate many of the general 
rinciples on which the progression of qua- 
io s is effected, we shall select the horse as 
an example. In standing, the animal rests its 
trunk on the legs which form the four columns 
of muppet. Let us suppose the legs to be 
placed on the ground at A B E D (fig. 245): 
if these points be joined, they form a rectangular 
parallelogram. , 
When the animal walks slowly with his 
right side in advance, the left hind leg moves 
first ; the right fore leg second ; the right hind 
leg third ; and the left fore leg fourth. During 
these four successive motions, the centre of 
gravity is propelled forwards over a space equal 
to the length of one step. Let us now investi- 
gate what takes place during these successive 
motions. The hind foot having been pre- 
viously extended, and having urged the centre 
of gravity forward, is first moved; it is then 
flexed, lifted from the ground, and advanced 
from E toe. Whilst the leg E is in the act of 
advancing to e, the trunk is supported on 
three legs, A B D, thus having the base 
of support transferred from the plane of a rec- 
tangular parallellogram to that of a right-angled 
triangle. 
In this movement, the centre of gravity G 
must fall within the plane A B D, which is 
effected by an oblique lateral movoment of the 
trunk towards B D. The foot E having taken 
a new position at e, the second foot B is set 
in motion, raised, and advanced to 6, and the 
base of support becomes transferred from the 
rectangular triangle, A B D, to the oblique- 
angled triangle, Ae D: by an oblique lateral 
motion of the trunk, the centre of gravity is 
propelled towards A a, within the new base of 
support, leaving the leg free to move without 
danger of the horse falling. The leg B having 
advanced and taken a new position at b, the 
Teg D is next raised and advanced to d, 
during which the base of support is transferred 
from the plane A e D, to that of A be, 
within which the centre of gravity is propelled ; 
lastly, the leg A is.advanced to a, and thie 
base of support is transferred from the plane 
A be to that of bed. When the leg A 
has reached the point a, the base of support 
becomes a new parallelogram a b e d, equal 
jn dimensions to thatof AB ED. In walk- 
MOTION. 
Fig. 245. 
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E (i D 
ing, the four legs move in the order above- 
mentioned successively. The time occupied it 
performing the series of movements to com- 
plete a step varies. In horses of large dimen- 
sious, one foot moves the length of a step ev 
second, and, therefore, each leg swings one qu 
ter, and rests on the ground three-quarters 0 
asecond. In walking at a more rapid pac 
each leg moves in rather a less period, and 
interval between the setting down of one 
and the rising of the next vanishes. 
The trot.—In the trot the — 
pairs diagonally. If the legs A D (fig. 2 
be first raised and advanced, then B Row 
generally be raised the instant that A D reac 
the ground ; on the other hand, when the le 
B E are raised before those of A D reach 
ground, the trot approximates to the gallop 0 
two beats, the four legs being at the same t 
for a minute interval, above the plane of m 
tion. The bases of support in the trot are’ 
lines A D and B E alternately. The san 
ot 
