Given a sound, well-proportioned horse standing to "attention", or "collectedly" as masters of equitation are wont to term it, with head up and ears forward, the face will have a profile whose angle to the ground is about 45 degrees, and the weight will be equally distributed among the four supports (limbs). As a matter of observation, horses seldom adopt this exact position, rather choosing to advance one foot slightly in front of its fellow, despite the training which they may have undergone in "dressing " in a troop of horses.
Movement must have some point of departure, and we will assume that our subject is in the position which Barrier and Goubaux, Le Coq and Stillman, Hayes and Marey all assume as possible, though Captain Hayes alone, among the authorities named, while admitting the jiossibility of an attitude such as is assigned to the horse by Goubaux, adds, " I have never seen a horse adopt it". Without insisting upon minutiae we may suppose our horse to be standing as nearly "square" as a horse will. In any change of attitude the centre of gravity will be shifted, and recovered by obtaining a new base of support.
In the slow pace of walking there is no elevation of the centre of gravity, and consequently no danger of losing the equilibrium; but in the faster paces this clanger exists, as will be readily seen in the illustration of a horse extended at the gallop, with the head advanced to the utmost limit winch other conditions of its carriage will permit (fig. 529). The draught-horse (pushing, be it remembered, for it is not draught1) lowers his head (when not artificially restrained by the bearing-rein), and so brings forward the centre of gravity. With the advance of a limb a new base of support is obtained, and as long as the centre of gravity falls within the base of support, equilibrium is maintained. In raising a limb the resistance encountered is only that of its own weight, or pressure of the atmosphere, and propulsion of the animal above and in front of the perpendicular, line of the centre of gravity is brought about by straightening the limb against the immovable surface of the ground.
Diminished resistance, as in deep) ground, results in a lesser degree of propulsion, apart from the deterrents to progression which arise from suction and the additional weight of soil attached to the foot. A good example of propulsion by straightening of the limbs against a fixed object is that of the swimmer who touches, turns, and strikes off from the side of the bath; the wall being immovable (with the force at disposal), while the water is readily displaced.
1 The propulsion of a vehicle is brought about by a series of levers bent upon one another between a fixed and a movable point. These levers act against the ground, where the toe is placed, and the collar.
Fig. 529. - Equilibrium in the Gallop.
Through the columns of bones the propulsion is directed, and of these in the horse two sets are in operation. The limb producing a forward and upward movement being invariably directed backward and downward, must necessarily act in the direction stated.