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_. [Illustration: FIG. 122.--POSTERIOR LIMB, GIVING THE IMPULSE (AFTER G. COLIN). A, Commencement of pressure; B, centre of pressure; C, termination of pressure.] 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. [Illustration: FIG. 123.--NOTATION OF THE AMBLING GAIT IN THE HORSE (AFTER PROFESSOR MAREY).] Professor Marey has studied these paces by a similar method to that which he adopted for the walking of man, and which we have already described. He employed hollow balls fixed under the hoofs, and a registering apparatus with four styles, each corresponding to one of the limbs. The tracing obtained is rather complicated, since two sets of lines are found marked. But a notation similar to that of which we have spoken can be discovered, and its exact signification should now be determined. For this purpose, we have selected the most simple (see Fig. 123). We there see, placed in two superimposed lines, the pressure markings of the right feet (white bands), and of the left feet (gray bands). On the upper line are found those related to the fore-legs; the lower lines contain those associated with the hind-legs. It is, in brief, the superposition of two notations of the human walking movements. And seeing that, as we have previously pointed out, we may make a comparison between a quadruped and two men placed one behind the other, it is easy to understand the significance of the superimposed notations, if we accustom ourselves to look on them as the notations of two bipeds. To read these notations--that is, to learn to know what occurs at each of the movements of the pace--it is necessary, indeed, to remember that they should be examined in vertical sections; it is to each of these sections--of these vertical divisions--that each of the movements which we more particularly wish to analyze corresponds.