The third is relative to running, and has already been represented in Fig. 118. The pressures of the feet are separated by the times of suspension.
The fourth also represents running, but in this case more rapid and characterized by the shorter pressures, the slightly longer periods of suspension intervals, and the quicker succession of movements.
Before putting aside the indications relating to the walking movements of man--indications which it was necessary to give in order to render intelligible those which are connected with the paces of the horse--we have yet to fix the value of that which we call 'a step.'
It is generally admitted that a step is constituted by the series of movements which are produced between the corresponding phases of the action of one foot and that of the other--for example, between the moment at which the right foot commences its pressure on the ground and that at which the left foot commences its own. It is necessary to adopt here another method of looking at it, and to regard the preceding as being but a _half-step_. The step should then be defined as being constituted by the series of movements which are executed between two similar positions of the same foot--as, for example, between the commencement of a pressure of the right foot and the similar phase of the following pressure of the same foot. We shall soon understand the importance of this definition.
Before entering on the details of the paces of the horse, it is necessary to see how the limbs of the latter oscillate during the period of a complete step; or, which is the same thing, to determine what the displacements are which a limb executes between two similar positions of its foot.
If we examine one of the limbs during a forward movement of the animal, we see that this limb passes through two principal phases: (1) It is raised from the ground; (2) it resumes contact with the ground. Each of these phases is divided into three periods of time, which we proceed to analyze in connection with the anterior limb.
[Illustration: FIG. 120.--SWING OF THE RAISED ANTERIOR LIMB (AFTER G. COLIN).[71]
[71] G. Colin, 'Traité de Physiologie Comparée des Animaux,' third edition, Paris, 1886.
The foot quits the ground (Fig. 120, C); this may be called _lifting_; the limb is oblique in direction downwards and backwards. This same limb is flexed and carried forward (Fig. 120, B), and, as it is supported by the action of its flexors, this is the period named _suspension_; the hoof is vertical. Then the limb is carried still further forward, becoming extended (Fig. 120, A); the heel is lowered, and the foot, being oblique, is directed towards the ground; this is the _placing_.