its solidity, will bear any amount of pressure to which it may be
subjected, the size, shape, and weight of animals destined to traverse
its surface are matters of little or no consequence. As, moreover, the
surface trod upon is rigid or unyielding, the extremities of quadrupeds
are, as a rule, terminated by small feet. Fig. 18 (contrast with
fig. 17).
[Illustration:
FIG. 18.--Chillingham Bull (_Bos Scoticus_). Shows powerful heavy
body, and the small extremities adapted for land transit. Also the
figure-of-8 movements made by the feet and limbs in walking and
running. _u_, _t_ Curves made by right and left anterior extremities.
_r_, _s_ Curves made by right and left posterior extremities. The
right fore and the left hind foot move together to form the waved
line (_s_, _u_); the left fore and the right hind foot move together
to form the waved line (_r_, _t_). The curves formed by the anterior
(_t_, _u_) and posterior (_r_, _s_) extremities form ellipses.
Compare with fig. 19, p. 39.--_Original._]
In this there is a double purpose--the limited area presented to the
ground affording the animal sufficient support and leverage, and
enabling it to disentangle its feet with the utmost facility, it
being a condition in rapid terrestrial progression that the points
presented to the earth be few in number and limited in extent, as
this approximates the feet of animals most closely to the wheel in
mechanics, where the surface in contact with the plane of progression
is reduced to a minimum. When the surface presented to a dense
resisting medium is increased, speed is diminished, as shown in the
tardy movements of the mollusc, caterpillar, and slowworm, and also,
though not to the same extent, in the serpents, some of which move
with considerable celerity. In the gecko and common house-fly, as
is well known, the travelling surfaces are furnished with suctorial
discs, which enable those creatures to walk, if need be, in an inverted
position; and “the tree-frogs (_Hyla_) have a concave disc at the end
of each toe, for climbing and adhering to the bark and leaves of trees.
Some toads, on the other hand, are enabled, by peculiar tubercles or
projections from the palm or sole, to clamber up old walls.”[21] A
similar, but more complicated arrangement, is met with in the arms of
the cuttle-fish.
[21] Comp. Anat. and Phys. of Vertebrates, by Professor Owen, vol. i.
pp. 262, 263. Lond. 1866.
The movements of the extremities in land animals vary considerably.
In the kangaroo and jerboa,[22] the posterior extremities only are
used, the animals advancing _per saltum_, _i.e._ by a series of
leaps.[23]
[22] The jerboa when pursued can leap a distance of nine feet, and
repeat the leaps so rapidly that it cannot be overtaken even by the
aid of a swift horse. The bullfrog, a much smaller animal, can, when
pressed, clear from six to eight feet at each bound, and project
itself over a fence five feet high.
[23] The long, powerful tail of the kangaroo assists in maintaining
the equilibrium of the animal prior to the leaps; the posterior
extremities and tail forming a tripod of support.
The deer also bounds into the air in its slower movements; in its
fastest paces it gallops like the horse, as explained at pp. 40–44.
The posterior extremities of the kangaroo are enormously developed
as compared with the anterior ones; they are also greatly elongated.
The posterior extremities are in excess, likewise, in the horse,
rabbit,[24] agouti, and guinea pig. As a consequence these animals
descend declivities with difficulty. They are best adapted for slightly
ascending ground. In the giraffe the anterior extremities are longer
and more powerful, comparatively, than the posterior ones, which is
just the opposite condition to that found in the kangaroo.
[24] The rabbit occasionally takes several short steps with the fore
legs and one long one with the hind legs; so that it walks with the
fore legs, and leaps with the hind ones.
In the giraffe the legs of opposite sides move together and alternate,
whereas in most quadrupeds the extremities move diagonally--a remark
which holds true also of ourselves in walking and skating, the right
leg and left arm advancing together and alternating with the left leg
and right arm (fig. 19).
[Illustration:
FIG. 19.--Diagram showing the figure-of-8 or double-waved track
produced by the alternating of the extremities in man in walking
and running; the right leg (_r_) and left arm (_s_) advancing
simultaneously to form one step; and alternating with the left
leg (_t_) and right arm (_u_), which likewise advance together to
form a second step. The continuous line (_r_, _t_) gives the waved
track made by the legs; the interrupted line (_s_, _u_) that made
by the arms. The curves made by the right leg and left arm, and by
the left leg and right arm, form ellipses. Compare with fig. 18,
p. 37.--_Original._]
In the hexapod insects, according to Müller, the fore and hind foot
of the one side and the middle one of the opposite side move together
to make one step, the three corresponding and opposite feet moving
together to form the second step. Other and similar combinations are
met with in the decapods.
The alternating movements of the extremities are interesting as
betokening a certain degree of flexuosity or twisting, either in the
trunk or limbs, or partly in the one and partly in the other.
This twisting begets the figure-of-8 movements observed in walking,
swimming, and flying. (Compare figs. 6, 7, and 26 _x_, pp. 28 and
55; figs. 18 and 19, pp. 37 and 39; figs. 32 and 50, pp. 68 and 97;
figs. 71 and 73, p. 144; and fig. 81, p. 157.)
_Locomotion of the Horse._--As the limits of the present volume
forbid my entering upon a consideration of the movements of all the
animals with terrestrial habits, I will describe briefly, and by
way of illustration, those of the horse, ostrich, and man. In the
horse, as in all quadrupeds endowed with great speed, the bones of
the extremities are inclined obliquely towards each other to form
angles; the angles diminishing as the speed increases. Thus the angles
formed by the bones of the extremities with each other and with the
scapulæ and iliac bones, are less in the horse than in the elephant.
For the same reason they are less in the deer than in the horse. In
the elephant, where no great speed is required, the limbs are nearly
straight, this being the best arrangement for supporting superincumbent
weight. The angles formed by the different bones of the wing of the
bird are less than in the fleetest quadruped, the movements of wings
being more rapid than those of the extremities of quadrupeds and
bipeds. These are so many mechanical adaptations to neutralize shock,
to increase elasticity, and secure velocity. The paces of the horse
are conveniently divided into the walk, the trot, the amble, and the
gallop. If the horse begins his walk by raising his near fore foot,
the order in which the feet are lifted is as follows:--first the left
fore foot, then the right or diagonal hind foot, then the right fore
foot, and lastly the left or diagonal hind foot. There is therefore
a twisting of the body and spiral overlapping of the extremities of
the horse in the act of walking, in all respects analogous to what
occurs in other quadrupeds[25] and in bipeds (figs. 18 and 19, pp. 37
and 39). In the slowest walk Mr. Gamgee observes “that three feet are
in constant action on the ground, whereas in the free walk in which
the hind foot passes the position from which the parallel fore foot
moves, there is a fraction of time when only two feet are upon the
ground, but the interval is too short for the eye to measure it. The
proportion of time, therefore, during which the feet act upon the
ground, to that occupied in their removal to new positions, is as
three to one in the slow, and a fraction less in the fast walk. In the
fast gallop these proportions are as five to three. In all the paces
the power of the horse is being exerted mainly upon a fore and hind
limb, with _the feet implanted in diagonal positions_. There is also a
constant parallel line of positions kept up by a fore and hind foot,
_alternating sides_ in each successive move. These relative positions
are renewed and maintained. Thus each fore limb assumes, as it alights,
the advanced position parallel with the hind, just released and moving;
the hind feet move by turns, in sequence to their diagonal fore, and
in priority to their parallel fellows, which following they maintain
for nearly half their course, when the fore in its turn is raised and
carried to its destined place, the hind alighting midway. All the feet
passing over equal distances and keeping the same time, no interference
of the one with the other occurs, and each successive hind foot as it
is implanted forms a new diagonal with the opposite fore, the latter
forming the front of the parallel in one instant, and one of the
diagonal positions in the next: while in the case of the hind, they
assume the diagonal on alighting and become the terminators of the
parallel in the last part of their action.”
[25] If a cat when walking is seen from above, a continuous wave
of movement is observed travelling along its spine from before
backwards. This movement closely resembles the crawling of the
serpent and the swimming of the eel.
[Illustration:
FIG. 20.--Horse in the act of trotting. In this, as in all the
other paces, the body of the horse is levered forward by a diagonal
twisting of the trunk and extremities, the extremities describing a
figure-of-8 track (_s u_, _r t_). The figure-of-8 is produced by the
alternate play of the extremities and feet, two of which are always
on the ground (_a_, _b_). Thus the right fore foot describes the
curve marked _t_, the left hind foot that marked _r_, the left fore
foot that marked _u_, and the right hind foot that marked _s_. The
feet on the ground in the present instance are the left fore and the
right hind. Compare with figs. 18 and 19, pp. 37 and 39.--_Original._]
In the trot, according to Bishop, the legs move in pairs diagonally.
The same leg moves rather oftener during the same period in trotting
than in walking, or as six to five. The velocity acquired by moving the
legs in pairs, instead of consecutively, depends on the circumstance
that in the trot each leg rests on the ground during a short interval,
and swings during a long one; whilst in walking each leg swings a
short, and rests a long period. The undulations arising from the
projection of the trunk in the trot are chiefly in the vertical plane;
in the walk they are more in the horizontal.
The gallop has been erroneously believed to consist of a series of
bounds or leaps, the two hind legs being on the ground when the two
fore legs are in the air, and _vice versâ_, there being a period when
all four are in the air. Thus Sainbell in his “Essay on the Proportions
of Eclipse,” states “that the gallop consists of a repetition of
bounds, or leaps, more or less high, and more or less extended in
proportion to the strength and lightness of the animal.” A little
reflection will show that this definition of the gallop cannot be the
correct one. When a horse takes a ditch or fence, he gathers himself
together, and by a vigorous effort (particularly of the hind legs),
throws himself into the air. This movement requires immense exertion
and is short-lived. It is not in the power of any horse to repeat these
bounds for more than a few minutes, from which it follows that the
gallop, which may be continued for considerable periods, must differ
very materially from the leap.
The pace known as the amble is an artificial movement, produced by the
cunning of the trainer. It resembles that of the giraffe, where the
right fore and right hind foot move together to form one step; the
left fore and left hind foot moving together to form the second step.
By the rapid repetition of these movements the right and left sides of
the body are advanced alternately by a lateral swinging motion, very
comfortable for the rider, but anything but graceful. The amble is a
defective pace, inasmuch as it interferes with the diagonal movements
of the limbs, and impairs the continuity of motion which the twisting,
cross movement begets. Similar remarks might be made of the gallop if
it consisted (which it does not) of a series of bounds or leaps, as
each bound would be succeeded by a halt, or dead point, that could not
fail seriously to compromise continuous forward motion. In the gallop,
as in the slower movements, the horse has never less than two feet on
the ground at any instant of time, no two of the four feet being in
exactly the same position.
Mr. Gamgee, who has studied the movements of the horse very carefully,
has given diagrams of the walk, trot, and gallop, drawn to a scale of
the feet of a two-year-old colt in training, which had been walked,
trotted, and galloped over the ground for the purpose. The point he
sought to determine was the exact distance through which each foot was
carried from the place where it was lifted to that where it alighted.
The diagrams are reproduced at figures 21, 22, and 23. In figure 23
I have added a continuous waved line to indicate the alternating
movements of the extremities; Mr. Gamgee at the time he wrote[26]
being, he informs me, unacquainted with the figure-of-8 theory of
animal progression as subsequently developed by me. Compare fig. 23
with figs. 18 and 19, pp. 37 and 39; with fig. 50, p. 97; and with
figs. 71 and 73, p. 144.
[26] “On the Breeding of Hunters and Roadsters.” Prize
Essay.--Journal of Royal Agricultural Society for 1863.
[Illustration: FIG. 21. WALK.]
[Illustration: FIG. 22. TROT.]
[Illustration: FIG. 23. GALLOP.]
In examining figures 21, 22, and 23, the reader will do well to
remember that the _near_ fore and hind feet of a horse are the _left_
fore and hind feet; the _off_ fore and hind feet being the _right_ fore
and hind feet. The terms _near_ and _off_ are technical expressions,
and apply to the left and right sides of the animal. Another point to
be attended to in examining the figures in question, is the relation
which exists between the fore and hind feet of the near and off sides
of the body. In slow walking the near hind foot is planted behind the
imprint made by the near fore foot. In rapid walking, on the contrary,
the near hind foot is planted from six to twelve or more inches in
advance of the imprint made by the near fore foot (fig. 21 represents
the distance as eleven inches). In the trot the near hind foot is
planted from twelve to eighteen or more inches in advance of the
imprint made by the near fore foot (fig. 22 represents the distance
as nineteen inches). In the gallop the near hind foot is planted 100
or more inches in advance of the imprint made by the near fore foot
(fig. 23 represents the distance as 110-1/2 inches). The distance by
which the near hind foot passes the near fore foot in rapid walking,
trotting, and galloping, increases in a progressive ratio, and is due
in a principal measure to the velocity or momentum acquired by the mass
of the horse in rapid motion; the body of the animal carrying forward
and planting the limbs at greater relative distances in the trot than
in the rapid walk, and in the gallop than in the trot. I have chosen to
speak of the near hind and near fore feet, but similar remarks may of
course be made of the off hind and off fore feet.
“At fig. 23, which represents the gallop, the distance between two
successive impressions produced, say by the near fore foot, is eighteen
feet one inch and a half. Midway between these two impressions is
the mark of the near hind foot, which therefore subdivides the space
into nine feet and six-eighths of an inch, but each of these is again
subdivided into two halves by the impressions produced by the off fore
and off hind feet. It is thus seen that the horse’s body instead of
being propelled through the air by bounds or leaps even when going at
the highest attainable speed, acts on a system of levers, the mean
distance between the points of resistance of which is four feet six
inches. The exact length of stride, of course, only applies to that of
the particular horse observed, and the rate of speed at which he is
going. In the case of any one animal, the greater the speed the longer
is the individual stride. In progression, the body moves before a limb
is raised from the ground, as is most readily seen when the horse is
beginning its slowest action, as in traction.”[27]
[27] Gamgee in Journal of Anatomy and Physiology, vol. iii. pp. 375,
376.
At fig. 22, which represents the trot, the stride is ten feet one
inch. At fig. 21, which represents the walk, it is only five feet five
inches. The speed acquired, Mr. Gamgee points out, determines the
length of stride; the length of stride being the effect and evidence of
speed and not the cause of it. The momentum acquired in the gallop, as
already explained, greatly accelerates speed.
“In contemplating length of strides, with reference to the fulcra,
allowance has to be made for the length of the feet, which is to be
deducted from that of the strides, because the apex, or toe of the
horse’s hind foot forms the fulcrum in one instant, and the heel of
the fore foot in the next, and _vice versâ_. This phenomenon is very
obvious in the action of the human foot, and is remarkable also for the
range of leverage thus afforded in some of the fleetest quadrupeds, of
different species. In the hare, for instance, between the point of its
hock and the termination of its extended digits, there is a space of
upwards of six inches of extent of leverage and variation of fulcrum,
and in the fore limb from the _carpus_ to the toe-nails (whose function
in progression is not to be underrated) upwards of three inches of
leverage are found, being about ten inches for each lateral biped, and
the double of that for the action of all four feet. Viewed in this
way the stride is not really so long as would be supposed if merely
estimated from the space between the footprints.
“Many interesting remarks might be made on the length of the stride of
various animals; the full movement of the greyhound is, for instance,
upwards of sixteen feet; that of the hare at least equal; whilst that
of the Newfoundland dog is a little over nine feet.”[27]
_Locomotion of the Ostrich._--Birds have been divided by naturalists
into eight orders:--the _Natatores_, or Swimming Birds; the
_Grallatores_, or Wading Birds; the _Cursores_, or Running Birds; the
_Scansores_, or Climbers; the _Rasores_, or Scrapers; the _Columbæ_,
or Doves; the _Passeres_; and the _Raptores_, or Birds of Prey.
The first five orders have been classified according to their habits
and modes of progression. The _Natatores_ I shall consider when I come
to speak of swimming as a form of locomotion, and as there is nothing
in the movements of the wading, scraping, and climbing birds,[28] or
in the _Passeres_[29] or _Raptores_, requiring special notice, I shall
proceed at once to a consideration of the _Cursores_, the best examples
of which are the ostrich, emu, cassowary, and apteryx.
[28] The woodpeckers climb by the aid of the stiff feathers of their
tails; the legs and tail forming a firm basis of support.
[29] In this order there are certain birds--the sparrows and
thrushes, for example--which advance by a series of vigorous leaps;
the leaps being of an intermitting character.
The ostrich is remarkable for the great length and development of
its legs as compared with its wings (fig. 24). In this respect it is
among birds what the kangaroo is among mammals. The ostrich attains
an altitude of from six to eight feet, and is the largest living bird
known. Its great height is due to its attenuated neck and legs. The
latter are very powerful structures, and greatly resemble in their
general conformation the posterior extremities of a thoroughbred
horse or one of the larger deer--compare with fig. 4, p. 21. They
are expressly made for speed. Thus the bones of the leg and foot are
inclined very obliquely towards each other, the femur being inclined
very obliquely to the ilium. As a consequence the angles made by the
several bones of the legs are comparatively small; smaller in fact than
in either the horse or deer.
[Illustration:
FIG. 24.--Skeleton of the Ostrich. Shows the powerful legs, small
feet, and rudimentary wings of the bird; the obliquity at which the
bones of the legs and wings are placed, and the comparatively small
angles which any two bones make at their point of junction. _a_ Angle
made by femur with ilium. _b_ Angle made by tibia and fibula with
femur. _c_ Angle made by tarso-metatarsal bone with tibia and fibula.
_d_ Angle made by bones of foot with tarso-metatarsal bone. _r_ Bones
of wing inclined to each other at nearly right angles. Compare with
fig. 4, p. 21, fig. 26, p. 55, and fig. 27, p. 59.--Adapted from
Dallas.]
The feet of the ostrich, like those of the horse and deer, are reduced
to a minimum as regards size; so that they occasion very little
friction in the act of walking and running. The foot is composed of two
jointed toes,[30] which spread out when the weight of the body comes
upon them, in such a manner as enables the bird to seize and let go
the ground with equal facility. The advantage of such an arrangement
in rapid locomotion cannot be over-estimated. The elasticity and
flexibility of the foot contribute greatly to the rapidity of movement
for which this celebrated bird is famous. The limb of the ostrich, with
its large bones placed very obliquely to form a system of powerful
levers, is the very embodiment of speed. The foot is quite worthy of
the limb, it being in some respects the most admirable structure of its
kind in existence. The foot of the ostrich differs considerably from
that of all other birds, those of its own family excepted. Thus the
under portion of the foot is flat, and specially adapted for acting
on plane surfaces, particularly solids.[31] The extremities of the
toes superiorly are armed with powerful short nails, the tips of which
project inferiorly to protect the toes and confer elasticity when the
foot is leaving the ground. The foot, like the leg, is remarkable
for its great strength. The legs of the ostrich are closely set,
another feature of speed.[32] The wings of the ostrich are in a very
rudimentary condition as compared with the legs.[33] All the bones are
present, but they are so dwarfed that they are useless as organs of
flight. The angles which the bones of the wing make with each other,
are still less than the angles made by the bones of the leg. This
is just what we would _a priori_ expect, as the velocity with which
wings are moved greatly exceeds that with which legs are moved. The
bones of the wing of the ostrich are inclined towards each other at
nearly right angles. The wings of the ostrich, although useless as
flying organs, form important auxiliaries in running. When the ostrich
careers along the plain, he spreads out his wings in such a manner that
they act as balancers, and so enable him to maintain his equilibrium
(fig. 25). The wings, because of the angle of inclination which their
under surfaces make with the horizon, and the great speed at which
the ostrich travels, act like kites, and so elevate and carry forward
by a mechanical adaptation a certain proportion of the mass of the
bird already in motion. The elevating and propelling power of even
diminutive inclined planes is very considerable, when carried along
at a high speed in a horizontal direction. The wings, in addition
to their elevating and propelling power, contribute by their short,
rapid, swinging movements, to continuity of motion in the legs. No bird
with large wings can run well. The albatross, for example, walks with
difficulty, and the same may be said of the vulture and eagle. What,
therefore, appears a defect in the ostrich, is a positive advantage
when its habits and mode of locomotion are taken into account.
[30] The toes in the emu amount to three.
[31] Feet designed for swimming, grasping trees, or securing prey, do
not operate to advantage on a flat surface. The awkward waddle of the
swan, parrot, and eagle when on the ground affords illustrations.
[32] In draught horses the legs are much wider apart than in racers;
the legs of the deer being less widely set than those of the racer.
[33] In the apteryx the wings are so very small that the bird is
commonly spoken of as the “wingless bird.”