Popular Science Monthly/Volume 6/December 1874/The Paces of the Horse
IN the November Monthly we gave a brief account of Prof. Marey's method of representing the step of animals by means of graphic illustrations, with its application to human locomotion; we will now consider it as applied to the more complex paces of the horse. Hitherto, the locomotion of the horse has been mainly studied by means of the eye and ear. In the horse, even at a walk, the motions of the limbs are so confusing as to make it difficult for the untrained eye to follow them, and, when the pace is more rapid, the movements seem hopelessly intricate. Indeed, observation by the eye alone long since gave place to the use of the ear, which, taking account of the rhythm of the steps by the sounds they produce, afforded much more accurate results.
An expedient which greatly aided the observer, and which we shall find of service in explaining the results obtained by the graphic method, was to concentrate the attention on a single pair of limbs, instead of attempting to keep all four under observation at once. Any two limbs thus selected are called a biped, and this is designated according to the relative position of the limbs chosen. The horse may thus be parceled out into six different bipeds. The forward limbs constitute the anterior biped; the hind-limbs, the posterior biped; the two right limbs, the right lateral biped; the two left limbs, the left lateral biped; the right fore-leg and the left hind-leg, the "right diagonal biped"; the left fore-leg and the right hind-leg, the left diagonal biped. The horsey reader may dwell a moment upon this bit of equine technics, as it will materially assist him in understanding the explanation of the various paces.
The quadruped, when walking, has been compared to two men, placed one before the other, the hindmost following close upon the forward step of his companion. According as these persons (who ought both to take the same number of steps) move their limbs simultaneously, or alternately, according as the man in front executes his movements more quickly or more slowly than the one behind, we see reproduced all the rhythms of the movements which characterize the different paces of the horse. Many have seen in the circus the figures of animals whose legs are formed by those of two men, with their bodies concealed in what represents the body of the quadruped. This grotesque imitation bears a striking resemblance to the animal when the movements of the two men are so well coordinated as to reproduce the rhythms of the paces.
Assuming the horse to be composed of two bipeds walking one behind
the other, let these, in progressing steadily, go through the same movements at the same time; that is, let the right leg of each be advanced at the same time and rate, the feet striking the ground so as to give but a single sound. While the body is resting on these, the left legs are simultaneously thrown forward, each striking the ground at the same moment, and so on alternately. The pace thus produced is known as the amble, and is the simplest of all the paces of the horse. The notation of its rhythms is given in Fig. 1. The upper line is derived from the movements of the anterior, or foremost biped; the
lower, from the movements of the posterior or hindmost biped. The foot-falls of the right and left foot being produced at the same time by the biped walking in front, and by the one which follows, must be represented by similar signs placed exactly over each other. In the horse, this agreement between the movements of the fore and hind limbs belongs to the amble, and the notation is the same as would be given by that pace. In the amble, the ear perceives only two beats at each pace, the two limbs on the same side striking the ground at the same instant. In the notation, these two sounds are marked by vertical lines joining the two synchronous impacts. In the amble, the pressure of the body on the ground is said to be lateral, as the two limbs on one side only are in contact with the ground at the same time.
The rhythms of both the walk and the trot have been similarly ascertained and expressed, but beyond this the unaided senses have failed to give us much trustworthy information. It has been reserved for M. Marey to surmount the difficulties of the investigation; and we will now give, though necessarily in an imperfect way, some account of his methods and results.
|Fig. 3.—Apparatus to give the Signals of the Pressure and Rise of the Horse's Hoof.|
For the shoe employed in the experiments on man, M. Marey substitutes, in the case of the horse, a ball of India-rubber filled with horse-hair, and attached to the shoe on the under-side of the hoof. The contrivance is shown in Fig. 2. A strong band of India-rubber passes over the apparatus and keeps in its place the ball filled with horse-hair, allowing it to rise slightly above the lower surface of the shoe. When the foot strikes the ground, the ball is compressed, which drives a part of the confined air into the registering instruments. As the foot is raised the ball recovers its form, and again fills with air, to be expelled at the next impact of the foot on the ground. Another form of apparatus, serving substantially the same purpose, and better adapted to ordinary roads, is seen in Fig. 3. This consists of a kind of leather bracelet fastened by straps to the leg of the horse just above the fetlock-joint. In front of this bracelet, which furnishes a solid point of resistance, is firmly fixed a flat box of India-rubber; this box communicates by a transmission-tube with the registering apparatus. Every pressure exerted on the box moves the corresponding registering-lever. A plate of copper, inclined about 45°, is connected at its upper extremity with a kind of hinge, while its lower end is fastened by a heavy wire to the upper face of the India-rubber box, on which it presses by means of a flat disk. On a wire parallel to the slip of copper slides a ball of lead, the position of which can be varied so as to increase or diminish the pressure which this jointed apparatus exerts on the India-rubber box. This apparatus is called into action by the movements of the limb; the inclination of the oscillating portions allows them to act on the membrane constituting the wall of the box during the movement of elevation, of descent, and of horizontal progress of the foot.
The general arrangement of the apparatus, as it is applied to the horse, is seen in Fig. 4. Thick transmitting-tubes, not easily crushed, connect the experimental shoes, or instruments, on the legs, with the
registering apparatus in the hand of the rider. The registrar now carries a great number of levers; he must have four, at least, one for each of the legs, and usually two others, which receive their movements of reaction from the withers and the croup. The hand which holds the reins also carries a ball of India-rubber, which is connected by a tube with the registering instrument, and by means of which the tracings may be made to commence at any desired moment.
The tracings furnished by this apparatus, when the horse is at a full trot, and the notation of the rhythm of that pace, as derived from these tracings, are shown in Fig. 5. Above are the reactions taken from the withers for the fore-part of the animal, indicated by the line R A (anterior reactions), and from the croup for the hinder part, indicated by the line R P (posterior reactions). Below are given the curves of pressure of the four feet, drawn at two different levels: the uppermost are the curves of the anterior limbs; those below, of the posterior limbs. In each series the curves of the left foot are drawn with dotted lines, those of the right with full lines.
The moment when the curve begins its rise represents the commencement of the pressure of the foot on the ground; the point at which the curve begins to descend represents the moment when the rise of the foot commences. It is seen from these tracings that the feet AG and PD, left fore-foot and right hind-foot, strike the ground at the same time. The simultaneous lowering of the curves of the two feet shows that they also rise from the ground simultaneously. Under these curves is placed the notation which represents the pressure of the left diagonal biped. The second impact is given by the feet AD and PG (right diagonal biped), and so on through the whole length of the tracing. Thus the free trot is a pace in which all the four feet give but two strokes, and in which the ground is struck in turn by the two diagonal bipeds; it is also a high pace, the animal being raised for a brief interval between two successive strokes above the ground. The duration of this suspension, according to Fig. 5, is equal to half the time the feet are pressing on the ground. But the trot varies greatly in different horses in this particular, there being oftentimes a very slight period of suspension, although a perfect synchronism of the diagonal strokes of the feet is observed.
By comparing the lines illustrating the reactions with the tracings afforded by the movements of the limbs, it will be seen that the moment when the body of the animal is at the lowest part of its vertical oscillation coincides precisely with that at which its feet touch the ground. The time of suspension does not depend on the fact that the
We have learned that one of the chief characteristics of the free trot is the entire synchronism of the strokes of each diagonal biped. There is a form of this pace, however, called by M. Marey the irregular trot, where such synchronism is wanting, the hind limb of one or both diagonal bipeds striking the ground an instant later than the corresponding fore-limb. Fig. 6 represents the notation of the irregular trot. The stroke of the left fore-foot is seen to be a little earlier than that of the right hind-foot, and the same is true of the limbs belonging to the right diagonal biped.
The low and short trot is represented in Fig. 7. The diagonal impacts succeed each other without interval, as may be seen in the notation placed below the figure. The animal has been depicted from the notation. The instant which the artist has chosen is that marked in the notation by a white dot. At this moment, as the superposition indicates, the left fore-foot is at the end of its pressure; the right fore-foot is about to reach the ground; the right hind-foot is finishing its pressure, and the left hind-foot is about to fall.
The elevated and lengthened trot is represented in Fig. 8. The animal is depicted at the instant which in the notation is represented by a dot; that is to say, during the time of the suspension, at the moment when the left diagonal biped has just risen, and the right diagonal biped is about to descend.
|Fig. 9.—Tracings and Notations of the Walking-Pace, with Equal Pressures of the Feet, both diagonally and laterally.|
Tracings afforded by the walking-pace are shown in Fig. 9. If we let fall a perpendicular from the points at which the curves commence, we shall have the positions of the successive impacts of the four legs. The order of succession of impacts is represented by the letters AD, PG, AG, PD, that is to say, right fore-foot, left hind-foot, left fore-foot, right hind-foot. The notation of the rhythm of the pressure of each foot, as derived from the registered curves, shows that the interval which separates the impacts is the same throughout, and consequently that the horse rests during the same time on the lateral as on the diagonal bipeds. This, however, is not always the case, some horses resting longer on the lateral biped than on the diagonal, and vice versa. The change of position of the centre of gravity may be seen by reference to Fig. 9. From 1 to 2 the horse will rest on the right lateral biped; from 2 to 3 on the right diagonal biped (that is to say, on that in which the right foot comes first); from 3 to 4 on the left lateral biped; from 4 to 5 on the left diagonal biped; again, from 5 to 6 the horse would find himself, as at the beginning, on the right lateral biped.
Observations on draught-horses have shown that, when the animal strives to react against a load, he may have three feet on the ground at once. This is held by some to be the rule in the normal walking-pace, but M. Marey has proved to the contrary. The vertical oscillations of the walk are chiefly at the withers, those of the croup being very slight. The actions of the hinder parts seem to consist chiefly in a forward propulsion, with a scarcely perceptible impulsion of the body in an upward direction. This agrees with the theory quite generally admitted, that the fore-legs have little to do in the normal pace, except to support alternately the fore-part of the body, while to the hind-limbs belong the propulsive action and the tractive force exerted by the animal. Fig. 12 is a representation of the horse at a walking-pace. The instant is marked in the notation by a dot.
The gallop comprises all those paces in which irregular impacts of the feet upon the ground recur at regular intervals. Most writers distinguish three kinds of gallop by the rhythm of the impacts, and name them, according to this rhythm, gallop in two, three, and four time. The most common kind is the gallop in three-time, from which the tracings in Fig. 11 have been obtained. At the commencement of the figure the animal is suspended above the ground; then comes the impact P G, which announces that the left hind-foot touches the ground. This is the foot diagonally opposed to that which the horse places forward in the gallop, and whose impact A D will be the last produced. Between these two impacts and in the middle of the interval which separates them, comes the simultaneous impact of the two feet forming the left diagonal biped. The superposition of the notations A G, P D, clearly shows this synchronism. In this series of movements the ear has therefore distinguished three sounds at nearly equal intervals. The first sound is produced by a hinder-foot, the second by a diagonal biped, the third by a fore-foot. Between the single impact of a fore-foot, which constitutes the third sound, and the first beat of the pace which follows, there is a period of silence whose duration is exactly equal to that of the three impacts taken together; then the series of movements recommences.
By an inspection of the curves, we see that the pressure of the feet on the ground must he more energetic in the gallop than in the paces already described, the height of the curves being greater than for either the trot or the walk. The greatest energy seems to belong to the first impact. At this moment, the body, raised for an instant from the ground, falls again, and one leg alone sustains the shock. The notation, Fig. 12, enables us to follow (in A) the succession of impacts; and shows (in B) the succession of the limbs which cause these pressures on the ground. The reactions of this pace, produced at the withers, are seen in Fig. 11 (R). There is an undulatory elevation, which lasts all the time that the animal touches the ground; in
this elevation are recognized the effects of the three impacts, which give it a triple undulation. The minimum elevation of the curve corresponds, as in the trot, with the moment when the feet do not touch the ground. Therefore, it is not a projection of the body into the air, which constitutes the time of suspension of the gallop. By comparing the reactions of this pace with those of the trot (Fig. 5), we see that in the gallop the rise and fall of the body are effected in a less sudden
The attitude of the horse at the moment of the first beat of the gallop in three-time is given in Fig. 13. The left hind-foot, on which the horse has just descended, alone rests on the ground.
Fig. 14 is the position of the horse at the time of the second beat, or at the moment when the left diagonal biped has finished its impact; the right fore-foot is about to reach the ground, the left hind-foot has just risen.
At the third or last beat of the pace, the position of the animal is that given in Fig. 15. The moment chosen is that in which the right foot alone rests on the ground, and is about to rise in its turn.
The gallop in four-time differs from that which has just been described only in this particular, that the impacts of the diagonal biped, which constitute the second beat of that gallop, are in this case disunited and give distinct sounds. This is shown in Fig. 16. According to this notation, the body, at first suspended, is borne successively on one foot, on three, on two, on three, and on one, after which a new suspension commences.
The full gallop, which is a very rapid pace, is in four-time. The impacts of the hinder-limbs, however, follow each other at such short intervals, that the ear can only distinguish one of them; but those of the fore-legs are notably more separated, and can be heard distinctly as two sounds. The notation of the full gallop (Fig. IV) confirms this. Another character of the full gallop is, that the longest period of silence takes place during the pressure of the hinder-limbs. The time of suspension appears to be extremely short. The reactions in the full gallop reproduce with great exactness the rhythm of the impacts. Thus it is observed that, at the moment of the almost synchronous impacts of the two hinder limbs, there is a sharp and prolonged reaction, after which two less sudden reactions take place, each of which corresponds with the impact of one of the fore-feet. The irregular line in Fig. 17 is the tracing; of the reactions at the withers.
Many other points relating to the locomotion of the horse, such as the characters of the footprints belonging to each pace; the transition from one pace to another; the modification of the movements incident to pulling a load, etc., we are unable to notice here, and would therefore refer the interested reader to M. Marey's work, where he will find the subject fully elucidated.
- Abstract of Chapters IV., V., and VI., of "Animal Mechanism," by Prof. Marey. (Vol. XI. of "The International Scientific Series.")