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] C, Lifting; B, suspension; A, placing.] [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_. Then takes place pressure (Fig. 121). The foot has just been placed on the ground; the limb is oblique in direction downwards and forwards; this we call _commencement of the pressure_ (Fig. 121, A). Then the body, being carried forward, whilst the hoof, D, is fixed on the ground, the limb becomes vertical: this stage is _mid-pressure_ (Fig. 121, B). Finally, the progression of the body continuing, the limb becomes oblique downwards and backwards; it is now at the _termination of pressure_ (Fig. 121, C), and proceeds to lift itself anew if another step is to be made. In conclusion, the inferior extremity of the limb describes, from its elevation to its being placed on the ground, an arc of a circle around its superior extremity (Fig. 121, D); whilst, during the pressure, it is its superior extremity which describes one around its inferior extremity, then fixed on the ground (Fig. 121, D). [Illustration: FIG. 121.--SWING OF THE ANTERIOR LIMB ON THE POINT OF PRESSURE (AFTER G. COLIN). A, Commencement of the pressure; B, centre of the pressure; C, termination of the pressure.] If we simultaneously examine the two fore-limbs, we remark that when one of them begins its pressure the other ends it, and _vice versâ_. 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.