The method of Professor Marey rests on the following principle: Suppose two rubber globes connected with one another by a tube. If we compress one of these globes, the air which it contains will be driven into the other, and will afterwards return when the pressure has ceased. Nothing more simple, evidently; but it is necessary to describe it in detail in order the better to comprehend that which follows: The walker who is the subject of experiment is furnished with special shoes (Fig. 115), having thick indiarubber soles, hollowed in the interior, so that the whole thus const.i.tuted forms a sort of hollow cushion which is compressed under the influence of the pressure of the foot on the ground. A tube which is attached to a registering apparatus, which the person who is walking carries in his hand, communicates with this cavity (Fig. 116).

This apparatus is formed of a metal drum, which is closed at its upper part by a flexible membrane. Each time that one of the man's feet presses on the ground, the air contained in the cavity of the sole of the shoe is driven into the drum, which we have just mentioned, and the flexible membrane of this drum is elevated. To this membrane is attached a vertical rod which supports a horizontal style.

[Ill.u.s.tration: FIG. 115.--EXPERIMENTAL SHOES, INTENDED TO RECORD THE PRESSURE OF THE FOOT ON THE GROUND.]

When the membrane, as we have just seen, is elevated, the style is lifted, and then descends when the pressure of the foot ceases. It traces these displacements on a leaf of paper, the surface of which is covered with a thin layer of lamp-black, which it removes by its contact; different parts of this surface are successively presented to it, the paper being rolled round a cylinder which is turned on its axis by means of a clockwork movement. It is necessary to add that the inscription is made, in the study of the walk of man, by means of two styles, each corresponding to one of the feet.

The tracings thus obtained, which are read from left to right, are sufficiently simple; but to understand them properly, it is necessary to remember that the style undergoes a movement of ascensional displacement during each pressure of a foot, and that, on the other hand, it descends when the latter is separated from the ground. We also see, on the tracing which it leaves, a line which ascends and then descends; the meaning of this is that first the foot presses on the ground, and is afterwards raised from it.

[Ill.u.s.tration: FIG. 116.--RUNNER FURNISHED WITH THE EXPLORATORY AND REGISTERING APPARATUS OF THE VARIOUS PACES.]

On the tracing (Fig. 117), the line D relates to the right foot; the line G, which is dotted so that it may not be confused with the preceding, corresponds to the left foot. The line G first ascends; the meaning of which is that the left foot presses on the ground; afterwards it descends: this indicates that the pressure of the foot has ceased. It is the same for the right foot. As we see, the pressures succeed each other; when the left foot touches the ground, the right is separated from it; when the latter presses the ground, it is the left which no longer rests there.

The line O is related to the movements of the body, as indicated by the oscillations of the head. We will neglect these.

But this tracing, which serves us for an example, is not, it must indeed be said, of very easy reading; it would be still less so if the paces of a horse were registered, for there would then be four lines, the entanglement of which would cause greater complication.

These difficulties of reading need be no longer feared, if we transform the tracing into a notation by means of the following diagram.

There are drawn (Fig. 118) below the graphic tracing two horizontal lines (1, 2). From the point where the line D rises (commencement of the pressure of the right foot), and from the point where this same line descends (end of the same pressure), we let fall two vertical lines joining the two horizontal ones mentioned above. At this plane, and between the two vertical lines, we mark a broad white one (_a, b_). This expresses, by its length, the duration of the period of pressure of the right foot. In doing the same for the line G, we obtain for the indication of a pressure of the left foot an interval of the same kind, in which are marked cross-lines, or which is tinted gray, in order to avoid all confusion with the preceding tracing.

[Ill.u.s.tration: FIG. 117.--TRACING OF THE RUNNING OF A MAN (AFTER PROFESSOR MAREY.)

D, Pressures and elevations of the right foot; G, pressures and elevations of the left foot.]

This notation can, with sufficient exact.i.tude, be compared to that which is employed in the musical scale. The horizontal lines 1 and 2 represent the _compa.s.s_. We there also see _notes_; these are the bars indicating the pressure, of which the value--that is to say, the duration--is represented by the length of these bars. It is the same with regard to the intervals of _silence_: these are expressed by the intervals which separate the pressures, and correspond to the moments in which, during certain paces, such as running, the body is raised from the ground.

Besides, we see intervals of this kind on the notation reproduced (Fig.

118) relative to the running of man.

[Ill.u.s.tration: FIG. 118.]

[Ill.u.s.tration: FIG. 119.]

In order to make the signification of these tracings still better understood, we reproduce four varieties of them (Fig. 119).

The first notation is that of ordinary walking. The pressures succeed each other regularly.

The second shows what takes place during the ascent of a staircase. At a certain moment, the weight of the body is upon both feet at the same time, one of them not quitting the lower step, until the other is already in contact with the step above. Accordingly, there is thus produced an overriding of the pressures.

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 const.i.tuted 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 const.i.tuted 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 pa.s.ses through two princ.i.p.al 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 a.n.a.lyze in connection with the anterior limb.

[Ill.u.s.tration: FIG. 120.--SWING OF THE RAISED ANTERIOR LIMB (AFTER G.

COLIN).[71]

C, Lifting; B, suspension; A, placing.]

[71] G. Colin, 'Traite de Physiologie Comparee 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).

[Ill.u.s.tration: 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 versa_.

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 pa.s.sing 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 const.i.tutes 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_.

[Ill.u.s.tration: 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 a.s.sociation 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.

[Ill.u.s.tration: 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 b.a.l.l.s 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 a.s.sociated 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 a.n.a.lyze corresponds.

We proceed to study first the pace of ambling, because it is the most simple; we shall then consider the trot, and, finally, we shall examine that which is the most complicated, viz., the step.