Fig. 122.—Posterior Limb, giving the Impulse (after G. Colin).
A, Commencement of pressure; B, centre of pressure; C, termination of pressure.
As to the hind-limbs, the oscillations are similar to those of the fore ones. In the second half of the pressure—that is, when they are passing from the vertical direction ([Fig. 122], A) to extreme obliquity backwards ([Fig. 122], C)—the effect of their action is to give propulsion to the body.
The fore and hind limbs make the same number of steps, and the steps have the same length.
The limbs of any quadruped—but we make special allusion to those of the horse—are divided into groups in the following manner:
The anterior pair constitutes the anterior biped. The posterior biped is that formed by the posterior limbs.
The name of lateral biped serves to designate the whole formed by the two limbs of the same side. The right fore-limb and the right hind-limb form the right lateral biped. The two others form the left lateral biped.
A fore-limb and hind-limb belonging to the opposite side form a diagonal biped, which also takes the name of the fore-limb which forms a part of it. Thus, the right diagonal biped is formed by the association of the right fore-limb and the left hind one. The left diagonal biped is, consequently, the inverse.
It is necessary to remember well these preliminary indications; it is the only means of comprehending with facility that which is about to follow.
Let us first return to the grouping of the limbs. The denominations anterior and posterior bipeds render clearly perceptible the comparison which consists in regarding a horse when walking as capable of being represented by two men marching one behind the other, and making the same number of steps. According as they move the legs of the same side at the same time in ‘covering the step,’ or march in contretemps step, we find reproduced all the rhythms which characterize the different paces of the horse.