Mode of Action, etc._--If time and space had permitted, I would have
considered it my duty to describe, more or less fully, the muscular
arrangements of all the animals whose movements I propose to analyse.
This is the more desirable, as the movements exhibited by animals of
the higher types are directly referable to changes occurring in their
muscular system. As, however, I could not hope to overtake this task
within the limits prescribed for the present work, I shall content
myself by merely stating the properties of muscles; the manner in
which muscles act; and the manner in which they are grouped, with a
view to moving the osseous levers which constitute the bony framework
or skeleton of the animals to be considered. Hitherto, and by common
consent, it has been believed that whereas a flexor muscle is situated
on one aspect of a limb, and its corresponding extensor on the other
aspect, these two muscles must be opposed to and antagonize each other.
This belief is founded on what I regard as an erroneous assumption,
viz., that muscles have only the power of shortening, and that when
one muscle, say the flexor, shortens, it must drag out and forcibly
elongate the corresponding extensor, and the converse. This would be a
mere waste of power. Nature never works against herself. There are good
grounds for believing, as I have stated elsewhere,[14] that there is
no such thing as antagonism in muscular movements; the several muscles
known as flexors and extensors; abductors and adductors; pronators
and supinators, being simply correlated. Muscles, when they act,
operate upon bones or something extraneous to themselves, and not upon
each other. The muscles are folded round the extremities and trunks
of animals with a view to operating in masses. For this purpose they
are arranged in cycles, there being what are equivalent to extensor
and flexor cycles, abductor and adductor cycles, and pronator and
supinator cycles. Within these muscular cycles the bones, or extraneous
substances to be moved, are placed, and when one side of a cycle
shortens, the other side elongates. Muscles are therefore endowed with
a centripetal and centrifugal action. These cycles are placed at every
degree of obliquity and even at right angles to each other, but they
are so disposed in the bodies and limbs of animals that they always
operate consentaneously and in harmony. _Vide_ fig. 5, p. 25.
[14] “Lectures on the Physiology of the Circulation in Plants, in the
Lower Animals, and in Man.”--Edinburgh Medical Journal for January
and February 1873.
There are in animals very few simple movements, _i.e._ movements
occurring in one plane and produced by the action of two muscles.
Locomotion is for the most part produced by the consentaneous action
of a great number of muscles; these or their fibres pursuing a variety
of directions. This is particularly true of the movements of the
extremities in walking, swimming, and flying.
Muscles are divided into the voluntary, the involuntary, and the
mixed, according as the will of the animal can wholly, partly, or in
no way control their movements. The voluntary muscles are principally
concerned in the locomotion of animals. They are the power which moves
the several orders of levers into which the skeleton of an animal
resolves itself.
The movements of the voluntary and involuntary muscles are essentially
wave-like in character, _i.e._ they spread from certain centres,
according to a fixed order, and in given directions. In the extremities
of animals the centripetal or converging muscular wave on one side of
the bone to be moved, is accompanied by a corresponding centrifugal or
diverging wave on the other side; the bone or bones by this arrangement
being perfectly under control and moved to a hair’s-breadth. The
centripetal or converging, and the centrifugal or diverging waves of
force are, as already indicated, correlated.[15] Similar remarks may
be made regarding the different parts of the body of the serpent
when creeping, of the body of the fish when swimming, of the wing of
the bird when flying, and of our own extremities when walking. In all
those cases the moving parts are thrown into curves or waves definitely
correlated.
[15] Muscles virtually possess a pulling and pushing power; the
pushing power being feeble and obscured by the flaccidity of the
muscular mass. In order to push effectually, the pushing substance
must be more or less rigid.
It may be broadly stated, that in every case locomotion is the result
of the opening and closing of opposite sides of muscular cycles. By
the closing or shortening, say of the flexor halves of the cycles, and
the opening or elongation of the extensor halves, the angles formed by
the osseous levers are diminished; by the closing or shortening of the
extensor halves of the cycles, and the opening or elongation of the
flexor halves, the angles formed by the osseous levers are increased.
This alternate diminution and increase of the angles formed by the
osseous levers produce the movements of walking, swimming, and flying.
The muscular cycles of the trunk and extremities are so disposed with
regard to the bones or osseous levers, that they in every case produce
a maximum result with a minimum of power. The origins and insertions of
the muscles, the direction of the muscles and the distribution of the
muscular fibres insure, that if power is lost in moving a lever, speed
is gained, there being an apparent but never a real loss. The variety
and extent of movement is secured by the obliquity of the muscular
fibres to their tendons; by the obliquity of the tendons to the bones
they are to move; and by the proximity of the attachment of the muscles
to the several joints. As muscles are capable of shortening and
elongating nearly a fourth of their length, they readily produce the
precise kind and degree of motion required in any particular case.[16]
[16] The extensor muscles preponderate in mass and weight over the
flexors, but this is readily accounted for by the fact, that the
extensors, when limbs are to be straightened, always work at a
mechanical disadvantage. This is owing to the shape of the bones,
the conformation of the joints, and the position occupied by the
extensors.
[Illustration:
FIG. 6.--Wing of bird. Shows how the bones of the arm (_a_), forearm
(_b_), and hand (_c_), are twisted, and form a conical screw. Compare
with Figs. 7 and 8.--_Original._]
[Illustration: FIG. 7. FIG. 8.
FIG. 7.--Anterior extremity of elephant. Shows how the bones
of the arm (_q_), forearm (_q´x_), and foot (_o_), are twisted to
form an osseous screw. Compare with Figs. 6 and 8.--_Original._
FIG. 8.--Cast or mould of the interior of the left ventricle of the
heart of a deer. Shows that the left ventricular cavity is conical
and spiral in its nature. _a_ Portion of right ventricular cavity;
_b_, base of left ventricular cavity; _x_, _y_, spiral grooves
occupied by the spiral _musculi papillares_; _j_, _q_, spiral ridges
projecting between the _musculi papillares_. Compare with Figs. 6 and
7.--_Original._]
The force of muscles, according to the experiments of Schwann,
increases with their length, and _vice versa_. It is a curious
circumstance, and worthy the attention of those interested in
homologies, that the voluntary muscles of the superior and inferior
extremities, and more especially of the trunk, are arranged in
longitudinal, transverse, and oblique spiral lines, and in layers or
strata precisely as in the ventricles of the heart and hollow muscles
generally.[17] If, consequently, I eliminate the element of bone from
these several regions, I reproduce a typical hollow muscle; and what is
still more remarkable, if I compare the bones removed (say the bones of
the anterior extremity of a quadruped or bird) with the cast obtained
from the cavity of a hollow muscle (say the left ventricle of the heart
of the mammal), I find that the bones and the cast are twisted upon
themselves, and form elegant screws, the threads or ridges of which run
in the same direction. This affords a proof that the involuntary hollow
muscles supply the type or pattern on which the voluntary muscles are
formed. Fig. 6 represents the bones of the wing of the bird; fig. 7 the
bones of the anterior extremity of the elephant; and fig. 8 the cast
or mould of the cavity of the left ventricle of the heart of the deer.
[17] “On the Arrangement of the Muscular Fibres in the Ventricles
of the Vertebrate Heart, with Physiological Remarks,” by the
Author.--Philosophical Transactions, 1864.
“On the Muscular Arrangements of the Bladder and Prostate, and
the manner in which the Ureters and Urethra are closed,” by the
Author.--Philosophical Transactions, 1867.
“On the Muscular Tunics in the Stomach of Man and other Mammalia,” by
the Author.--Proceedings Royal Society of London, 1867.
[Illustration: FIG. 9.--The Superficial Muscles in the Horse, (after
Bagg).]
It has been the almost invariable custom in teaching anatomy, and
such parts of physiology as pertain to animal movements, to place
much emphasis upon the configuration of the bony skeleton as a whole,
and the conformation of its several articular surfaces in particular.
This is very natural, as the osseous system stands the wear and tear
of time, while all around it is in a great measure perishable. It is
the link which binds extinct forms to living ones, and we naturally
venerate and love what is enduring. It is no marvel that Oken, Goethe,
Owen, and others should have attempted such splendid generalizations
with regard to the osseous system--should have proved with such cogency
of argument that the head is an expanded vertebra. The bony skeleton
is a miracle of design very wonderful and very beautiful in its way.
But when all has been said, the fact remains that the skeleton, when
it exists, forms only an adjunct of locomotion and motion generally.
All the really essential movements of an animal occur in its soft
parts. The osseous system is therefore to be regarded as secondary
in importance to the muscular, of which it may be considered a
differentiation. Instead of regarding the muscles as adapted to the
bones, the bones ought to be regarded as adapted to the muscles. Bones
have no power either of originating or perpetuating motion. This begins
and terminates in the muscles. Nor must it be overlooked, that bone
makes its appearance comparatively late in the scale of being; that
innumerable creatures exist in which no trace either of an external
or internal skeleton is to be found; that these creatures move freely
about, digest, circulate their nutritious juices and blood when
present, multiply, and perform all the functions incident to life.
While the skeleton is to be found in only a certain proportion of the
animals existing on our globe, the soft parts are to be met with in
all; and this appears to me an all-sufficient reason for attaching
great importance to the movements of soft parts, such as protoplasm,
jelly masses, involuntary and voluntary muscles, etc.[18] As the
muscles of vertebrates are accurately applied to each other, and to
the bones, while the bones are rigid, unyielding, and incapable of
motion, it follows that the osseous system acts as a break or boundary
to the muscular one,--and hence the arbitrary division of muscles
into extensors and flexors, pronators and supinators, abductors and
adductors. This division although convenient is calculated to mislead.
The most highly organized animal is strictly speaking to be regarded as
a living mass whose parts (hard, soft, and otherwise) are accurately
adapted to each other, every part reciprocating with scrupulous
exactitude, and rendering it difficult to determine where motion begins
and where it terminates. Fig. 9 shows the more superficial of the
muscular masses which move the bones or osseous levers of the horse,
as seen in the walk, trot, gallop, etc. A careful examination of these
carneous masses or muscles will show that they run longitudinally,
transversely, and obliquely, the longitudinal and transverse muscles
crossing each other at nearly right angles, the oblique ones tending to
cross at various angles, as in the letter X. The crossing is seen to
most advantage in the deep muscles.
[18] Lectures “On the Physiology of the Circulation in Plants, in the
Lower Animals, and in Man,” by the Author.--Edinburgh Medical Journal
for September 1872.
In order to understand the twisting which occurs to a greater or less
extent in the bodies and extremities (when present) of all vertebrated
animals, it is necessary to reduce the bony and muscular systems
to their simplest expression. If motion is desired in a dorsal,
ventral, or lateral direction only, a dorsal and ventral or a right
and left lateral set of longitudinal muscles acting upon straight
bones articulated by an ordinary ball-and-socket joint will suffice.
In this case the dorsal, ventral, and right and left lateral muscles
form _muscular cycles_; contraction or shortening on the one aspect
of the cycle being accompanied by relaxation or elongation on the
other, the bones and joints forming as it were the diameters of the
cycles, and oscillating in a backward, forward, or lateral direction
in proportion to the degree and direction of the muscular movements.
Here the motion is confined to two planes intersecting each other at
right angles. When, however, the muscular system becomes more highly
differentiated, both as regards the number of the muscles employed,
and the variety of the directions pursued by them, the bones and
joints also become more complicated. Under these circumstances, the
bones, as a rule, are twisted upon themselves, and their articular
surfaces present various degrees of spirality to meet the requirements
of the muscular system. Between the straight longitudinal muscles,
therefore, arranged in dorsal and ventral, and right and left lateral
sets, and those which run in a more or less transverse direction, and
between the simple joint whose motion is confined to one plane and the
ball-and-socket joints whose movements are universal, every degree of
obliquity is found in the direction of the muscles, and every possible
modification in the disposition of the articular surfaces. In the fish
the muscles are for the most part arranged in dorsal, ventral, and
lateral sets, which run longitudinally; and, as a result, the movements
of the trunk, particularly towards the tail, are from side to side
and sinuous. As, however, oblique fibres are also present, and the
tendons of the longitudinal muscles in some instances cross obliquely
towards the tail, the fish has also the power of tilting or twisting
its trunk (particularly the lower half) as well as the caudal fin. In
a mackerel which I examined, the oblique muscles were represented by
the four lateral masses occurring between the dorsal, ventral, and
lateral longitudinal muscles--two of these being found on either side
of the fish, and corresponding to the myocommas or “_grand muscle
latéral_” of Cuvier. The muscular system of the fish would therefore
seem to be arranged on a fourfold plan,--there being four sets of
longitudinal muscles, and a corresponding number of slightly oblique
and oblique muscles, the oblique muscles being spiral in their nature
and tending to cross or intersect at various angles, an arrest of the
intersection, as it appears to me, giving rise to the myocommas and to
that concentric arrangement of their constituent parts so evident on
transverse section. This tendency of the muscular fibres to cross each
other at various degrees of obliquity may also be traced in several
parts of the human body, as, for instance, in the deltoid muscle of
the arm and the deep muscles of the leg. Numerous other examples
of penniform muscles might be adduced. Although the fibres of the
myocommas have a more or less longitudinal direction, the myocommas
themselves pursue an oblique spiral course from before backwards and
from within outwards, _i.e._ from the spine towards the periphery,
where they receive slightly oblique fibres from the longitudinal
dorsal, ventral, and lateral muscles. As the spiral oblique myocommas
and the oblique fibres from the longitudinal muscles act directly and
indirectly upon the spines of the vertebræ, and the vertebræ themselves
to which they are specially adapted, and as both sets of oblique fibres
are geared by interdigitation to the fourfold set of longitudinal
muscles, the lateral, sinuous, and rotatory movements of the body and
tail of the fish are readily accounted for. The spinal column of the
fish facilitates the lateral sinuous twisting movements of the tail and
trunk, from the fact that the vertebræ composing it are united to each
other by a series of modified universal joints--the vertebræ supplying
the cup-shaped depressions or sockets, the intervertebral substance,
the prominence or ball.
The same may be said of the general arrangement of the muscles in the
trunk and tail of the Cetacea, the principal muscles in this case
being distributed, not on the sides, but on the dorsal and ventral
aspects. The lashing of the tail in the whales is consequently from
above downwards or vertically, instead of from side to side. The spinal
column is jointed as in the fish, with this difference, that the
vertebræ (especially towards the tail) form the rounded prominences or
ball, the meniscus or cup-shaped intervertebral plates the receptacles
or socket.
When limbs are present, the spine may be regarded as being ideally
divided, the spiral movements, under these circumstances, being thrown
upon the extremities by typical ball-and-socket joints occurring at the
shoulders and pelvis. This is peculiarly the case in the seal, where
the spirally sinuous movements of the spine are transferred directly to
the posterior extremities.[19]
[19] That the movements of the extremities primarily emanate from
the spine is rendered probable by the remarkable powers possessed by
serpents. “It is true,” writes Professor Owen (_op. cit._ p. 261),
“that the serpent has no limbs, yet it can outclimb the monkey,
outswim the fish, outleap the jerboa, and, suddenly loosing the close
coils of its crouching spiral, it can spring into the air and seize
the bird upon the wing.” ... “The serpent has neither hands nor
talons, yet it can outwrestle the athlete, and crush the tiger in the
embrace of its ponderous overlapping folds.” The peculiar endowments,
which accompany the possession of extremities, it appears to me,
present themselves in an undeveloped or latent form in the trunk of
the reptile.
The extremities, when present, are provided with their own muscular
cycles of extensor and flexor, abductor and adductor, pronator and
supinator muscles,--these running longitudinally and at various
degrees of obliquity, and enveloping the hard parts according to their
direction--the bones being twisted upon themselves and furnished
with articular surfaces which reflect the movements of the muscular
cycles, whether these occur in straight lines anteriorly, posteriorly,
or laterally, or in oblique lines in intermediate situations. The
straight and oblique muscles are principally brought into play in the
movements of the extremities of quadrupeds, bipeds, etc. in walking;
in the movements of the tails and fins of fishes, whales, etc. in
swimming; and in the movements of the wings of insects, bats, and
birds in flying. The straight and oblique muscles are usually found
together, and co-operate in producing the movements in question; the
amount of rotation in a part always increasing as the oblique muscles
preponderate. The combination of ball-and-socket and hinge-joints, with
their concomitant oblique and longitudinal muscular cycles (the former
occurring in their most perfect forms where the extremities are united
to the trunk, the latter in the extremities themselves), enable the
animal to present, when necessary, an extensive resisting surface the
one instant, and a greatly diminished and a comparatively non-resisting
one the next. This arrangement secures the subtlety and nicety of
motion demanded by the several media at different stages of progression.
[Illustration: FIG. 10. FIG. 11. FIG. 12. FIG. 13. FIG. 14.
FIG. 10.--Extreme form of compressed foot, as seen in the deer, ox,
etc., adapted specially for land transit.--_Original._
FIG. 11.--Extreme form of expanded foot, as seen in the
_Ornithorhynchus_, etc., adapted more particularly for
swimming.--_Original._
FIGS. 12 and 13.--Intermediate form of foot, as seen in the otter
(fig. 12), frog (fig. 13), etc. Here the foot is equally serviceable
in and out of the water.--_Original._
FIG. 14.--Foot of the seal, which opens and closes in the act of
natation, the organ being folded upon itself during the non-effective
or return stroke, and expanded during the effective or forward
stroke. Due advantage is taken of this arrangement by the seal when
swimming, the animal rotating on its long axis, so as to present the
lower portion of the body and the feet obliquely to the water during
the return stroke, and the flat, or the greatest available surface of
both, during the effective or forward stroke.--_Original._]
_The travelling surfaces of Animals modified and adapted to the medium
on or in which they move._--In those land animals which take to the
water occasionally, the feet, as a rule, are furnished with membranous
expansions extending between the toes. Of such the Otter (fig. 12),
Ornithorhynchus (fig. 11), Seal (fig. 14), Crocodile, Sea-Bear
(fig. 37, p. 76), Walrus, Frog (fig. 13), and Triton, may be cited.
The crocodile and triton, in addition to the membranous expansion
occurring between the toes, are supplied with a powerful swimming-tail,
which adds very materially to the surface engaged in natation. Those
animals, one and all, walk awkwardly, it always happening that when
the extremities are modified to operate upon two essentially different
media (as, for instance, the land and water), the maximum of speed is
attained in neither. For this reason those animals which swim the best,
walk, as a rule, with the greatest difficulty, and _vice versâ_, as the
movements of the auk and seal in and out of the water amply testify.
In addition to those land animals which run and swim, there are
some which precipitate themselves, parachute-fashion, from immense
heights, and others which even fly. In these the membranous expansions
are greatly increased, the ribs affording the necessary support in
the Dragon or Flying Lizard (fig. 15), the anterior and posterior
extremities and tail, in the Flying Lemur (fig. 16) and Bat (fig. 17,
p. 36).
[Illustration: FIG. 15. FIG. 16.
FIG. 15.--The Red-throated Dragon (_Draco hæmatopogon_, Gray) shows
a large membranous expansion (_b b_) situated between the anterior
(_d d_) and posterior extremities, and supported by the ribs. The
dragon by this arrangement can take extensive leaps with perfect
safety.--_Original._
FIG. 16.--The Flying Lemur _Galeopithecus volans_, Shaw. In the
flying lemur the membranous expansion (_a b_) is more extensive than
in the Flying Dragon (fig. 15). It is supported by the neck, back,
and tail, and by the anterior and posterior extremities. The flying
lemur takes enormous leaps; its membranous tunic all but enabling it
to fly. The Bat, _Phyllorhina gracilis_ (fig. 17), flies with a very
slight increase of surface. The surface exposed by the bat exceeds
that displayed by many insects and birds. The wings of the bat are
deeply concave, and so resemble the wings of beetles and heavy-bodied
short-winged birds. The bones of the arm (_r_), forearm (_d_), and
hand (_n, n, n_) of the bat (fig. 17) support the anterior or thick
margin and the extremity of the wing, and may not inaptly be compared
to the nervures in corresponding positions in the wing of the
beetle.--_Original._]
[Illustration:
FIG. 17.--The Bat (_Phyllorhina gracilis_, Peters). Here the
travelling-surfaces (_r d e f_, _a n n n_) are enormously increased
as compared with that of the land and water animals generally.
Compare with figures from 10 to 14, p. 34. _r_ Arm of bat; _d_
forearm of bat; _e f_, _n n n_ hand of bat.--_Original._]
Although no lizard is at present known to fly, there can be little
doubt that the extinct Pterodactyles (which, according to Professor
Huxley, are intermediate between the lizards and crocodiles) were
possessed of this power. The bat is interesting as being the only
mammal at present endowed with wings sufficiently large to enable it to
fly.[20] It affords an extreme example of modification for a special
purpose,--its attenuated body, dwarfed posterior, and greatly elongated
anterior extremities, with their enormous fingers and outspreading
membranes, completely unfitting it for terrestrial progression. It is
instructive as showing that flight may be attained, without the aid of
hollow bones and air-sacs, by purely muscular efforts, and by the mere
diminution and increase of a continuous membrane.
[20] The Vampire Bat of the Island of Bonin, according to Dr.
Buckland, can also swim; and this authority was of opinion that the
Pterodactyle enjoyed similar advantages.--Eng. Cycl. vol. iv. p. 495.
As the flying lizard, flying lemur, and bat (figs. 15, 16, and
17, pp. 35 and 36), connect terrestrial progression with aërial
progression, so the auk, penguin (fig. 46, p. 91), and flying-fish
(fig. 51, p. 98), connect progression in the water with progression in
the air. The travelling surfaces of these anomalous creatures run the
movements peculiar to the three highways of nature into each other,
and bridge over, as it were, the gaps which naturally exist between
locomotion on the land, in the water, and in the air.
PROGRESSION ON THE LAND.