of this part of the body is dealt with on another page; here we are
concerned only with the external form of the feet and legs. Aquatic
birds often have webbed feet, but not always. The Dipper, for example,
is a bird which lives largely on and under the water, but its feet
are not in the least like those of a Duck or Grebe. The webbed foot
presents us with at least two varieties. In the Pelican tribe (fig. 5)
the extreme of web-footedness is to be seen. Here all the toes (four)
are connected by a webbing. In the Duck only three of the toes are
webbed. Another kind of webbed foot is termed palmate. In the Coots,
for example, each toe is fringed with a broad membrane, but there is no
connection between the fringes of successive toes. The toes of birds
are apt to be differently disposed. In most birds (fig. 6) there are
three toes which are turned forwards, and one, the great toe (hallux),
which is turned backwards. But in the Trogons and others two toes are
turned forwards and two backwards, thus producing a very efficient
mechanism for holding on tightly to the bough of a tree, a mechanism
which is shared by that, in some other respects, bird-like lizard, the
chameleon. A foot of this kind is technically called ‘zygodactyle.’ A
singular modification of the foot is seen in the Kingfisher (fig. 7)
where the two middle toes are enclosed in the same fold of skin; this
is called ‘syngenesious.’
Skeleton.
A bird’s skeleton is wonderfully light and spongy in texture. It is
full of air (see below, p. 27), but deficient in marrow. Its entire
structure is pre-eminently suited to a flying creature, not only for
the above reasons, but because the heaviest part (the sternum) lies in
the middle, in the centre of gravity, and thus assists in preserving
the balance, like Blondin’s pole.
The Skull.
The skull of a bird is composed of a large number of separate bones,
which are very closely united in the adult bird, so much so that it
is next to impossible to recognise that they are distinct bones. The
bones are also thin and light, for to a flying animal any weight
forward would be most disadvantageous. The weight of the bird should
be, and is, concentrated in the middle of the body. We can divide
the skull into two regions: behind is the smooth, rounded brain-case
or cranium; in front is the face, which is largely ensheathed by the
beak. It is chiefly formed by the maxillary and nasal bones above, and
by the palatine and pterygoids below. The length of this part of the
skull is subject to great variation in different birds. In the Storks,
for instance, the face is extremely long, while in the Parrots it is
comparatively short.
Professor Huxley, about thirty years ago, proposed to classify birds
by the form of the bones of the palate. In the skull of the Hawk, it
will be seen that two bones lying in the front region of the palate
are fused with each other in the middle line, and to the type of skull
which is thus characterised the name ‘desmognathous’ was given. It is
found not only in the Hawks, but in a quantity of other birds; for
instance, in the Stork tribe, and in the Hornbills and Toucans. The
second form of skull distinguishes the gallinaceous birds; in them the
two maxillo-palatines remain unconnected, and the palate is therefore
in a way cleft; this is termed the ‘schizognathous’ skull. In the
finch tribe there is a slight modification of this, called, from the
Greek word for a finch, ‘ægithognathous.’ In these birds a median bone,
called the vomer, from the fact that the bone to which it corresponds
in the human skull is shaped somewhat like a plough-share, is truncated
in front, instead of tapering, as it does in the schizognathous skull
of the common fowl. There is a fourth variety, which marks out the
Ostrich tribe and the American Tinamous, in which the two pairs of
bones called the pterygoids and palatines do not, as they do in the
types of skull that have been hitherto considered, reach the middle
line of the skull, but are kept off from it by the vomers, which
extend backwards. The term ‘dromæognathous,’ or emu-like, is applied
to this form of skull. If the back of any bird’s skull be examined,
it will be noticed that just below the great hole or foramen, through
which the medulla passes to join the spinal cord in the canal of the
vertebral column, is a rounded, rather kidney-shaped boss. This is the
occipital condyle, by means of which the skull articulates with the
first vertebra. If you look at the same region in a mammal, you will
find that there are two of these, one on each side, though also below
the foramen magnum. This is one of the many points of structure that
distinguish a bird from a mammal and ally it to the reptiles; but it
must be remembered that in some reptiles there is a commencing division
of the single condyle into two.
The Vertebral Column.
Like all other backboned animals, birds have a chain of small bones
running along the back, and enclosing a canal in which runs the spinal
marrow. In most vertebrates some of the individual vertebræ in the
region of the hind limb, the sacral region, are somewhat intimately
fused together, forming a more solid structure for the support of the
pelvis. In birds the strong coupling of the vertebræ is more marked,
and extends to the dorsal region. The mechanical value of this to a
flying animal is clear; it is analogous to the tight coupling of an
express train, and prevents the back from bending from side to side
under the strain produced by the powerful movements of the muscles in
flight. The tail vertebræ show some curious modifications in different
birds. In the typical carinate bird, the last few vertebræ are fused
into a piece which is called the ‘plough-share bone,’ or ‘pygostyle.’
The name of this bone sufficiently indicates its shape; the expanded
end of the bone serves as a firm base, upon which rest the strong
tail feathers. Now, in the ostrich tribe there are no rectrices
comparable in size to those of the flying carinates. Here there is no
pygostyle, but the individual vertebræ are small and disconnected.
They are, however, few in number, whereas in the Archæopteryx they are
numerous, though, oddly enough, not so numerous altogether as are the
tail vertebræ of some flying birds. Each individual vertebra in the
Archæopteryx supports a pair of rectrices, which are thus arranged in
a series, and not in one row. A very distinctive peculiarity of the
vertebræ of birds is the saddle-shaped centrum. The centrum of the
vertebra is the solid piece which underlies the canal of the spinal
cord, the walls of the latter being formed by the neural arches, which
unite above to form a neural spine. In other vertebrates the centra
are flat (mammals), or procœlous (the concavity being forward), or
opisthocœlous (the concavity posterior), or amphicœlous (concave on
both sides). This latter form of vertebra is frequently met with in
archaic forms belonging to various groups. It occurs, for example,
in many fishes. Such reptiles as Hyperodapedon and the Geckos have
the same kind of vertebræ. Among birds there is no existing genus or
species which is to be thus characterised; but the extinct Ichthyornis
had clearly biconcave vertebræ.
Shoulder Girdle.
[Illustration: FIG. 8.--STERNUM OF SHRIKE.
_h_, ribs; 58, furcula; 52, coracoid; 59, anterior end of
sternum.]
This series of bones serves as the intermediary between the fore limb
and the vertebral column. It consists of three distinct elements. There
is, first of all, a sword-blade-like bone with sharp edges, which
lies along the vertebral column--the scapula. To the end of this is
firmly attached a somewhat shorter bone, which approaches its fellow
as it joins the sternum below; this bone is known as the coracoid (52,
fig. 8). The angle between these two bones is, in flying birds, a
considerable one, but in the ostrich tribe they are almost in the same
straight line; this is really connected with the power of flight, for
it has been shown by careful measurements that, in birds which still
have wings that bear every appearance of being functional, and yet are
not used for their legitimate purpose, the angle tends to approach
the obtusity of the scapula and coracoid of the Ostrich. Birds have,
besides these two bones, the merry-thought, or clavicle (58, fig. 8),
which corresponds to our collar-bone. Its two halves are generally
closely united to form one [**Symbol:]U-shaped or [**Symbol:]V-shaped
bone; but sometimes they are separate, and then more or less
rudimentary.
Wing.
[Illustration: FIG. 9.--WING OF NESTLING OPISTHOCOMUS. (After
Pycraft in ‘Natural Science.’)
The second digit (II) is free, being prolonged beyond ala
membrane (P.m.), and remiges 8–10 are not developed.]
[Illustration: FIG. 10.--WING OF YOUNG FOWL OF SAME AGE AS FIG. 9
(OF WING OF OPISTHOCOMOS). (After Pycraft in ‘Natural Science.’)
The hand is shorter, and not fitted to be a grasping organ.]
[Illustration: FIG. 11.--WING OF ADULT OPISTHOCOMUS. (After
Pycraft in ‘Natural Science.’)
The hand is smaller relatively to the forearm; _c_, the claw of
digit I, much reduced.]
[Illustration: FIG. 12.--WING OF HALF-GROWN OSTRICH. (After
Parker.)
I, II, III, digits; R., U., D.c.f., carpal bones; Mc.,
metacarpals.]
We must enter into the matter of wing a little more closely--it is so
important a feature of bird organisation. The wing, of course, although
it performs so different a _rôle_, is the exact equivalent of
the fore limb of mammals. We can easily recognise precisely the same
bones, though they are diminished in number, and often of a different
form. It will be noticed that in each case we can distinguish the
three bones forming the arm, and which are known as the humerus, the
radius, and ulna. The rest of the limb in the bird is not quite so
obviously like the hand of the mammal; but a little attention will show
that it is constructed upon a perfectly similar plan. The flexible
wrist of the mammal is made up of many small bones; the hand itself
is made up of a larger series still, of which those nearest to the
wrist are technically termed the metacarpals, and those which follow,
the phalanges. In many mammals there are five fingers; but there are
many which have less, and the extreme is reached in the horse, which
has to put up with a single finger and small rudiments of two others.
Now the bird is better off in the way of fingers than the horse, as it
has three fairly well-developed fingers, or rather two well developed
and one less perfect. The shortest finger corresponds to the thumb of
our hand. It is more freely movable than the others. The metacarpal
bones of the second and third fingers are firmly welded together, and
are long; each finger (as will be seen from a look at fig. 1, p. 4)
has one or two phalanges, as the case may be. Now in mammals the end
phalanx of each finger is tipped with a nail, or with a hoof. The
powerful claws of the tiger, used for tearing, and the solid hoof of
the ox or horse, upon which the creature walks, are one and the same
thing. It might be supposed that the hand of the bird, which is not
an organ of offence or meant to walk with, might be shorn of these
appendages. But this is not the case: every bird has at least two nails
(fig. 9), of a long and rather claw-like form when well developed,
and sometimes three nails, that is, one to each of its fingers. It
looks, therefore, very much as if the wing of the bird had been formed
out of a limb that was once an organ for climbing or walking with.
There is a curious bird, found in British Guiana, which is known as
the Hoatzin (figs. 9, 11). In the very young nestlings of the hoatzin
the claws of the fingers are so conspicuous that they are actually
used by the callow chick to climb with, before the feathers of the
wings have grown sufficiently to enable them to use their wings in the
proper way in which a bird should; it has been said also, that other
birds scramble about and use their claws when they are young. In the
case of the hoatzin, it is stated that the thumb and the first finger
can be brought together so as to lay hold definitely of an object. A
very important thing to notice about the wing bones is that they are
capable of but little movement upon each other. There are two hinges,
one at the elbow, and the other at the wrist; but the radius and ulna
cannot move round each other, as they can in our arms, and the fingers
are fixed and rigid. This would be most unfortunate if the wing had
to be used as a walking or climbing limb; but it is most useful in
relation to the function which the wing has to perform--that of flight.
The strength of the downward stroke would be enfeebled if the bones
were in a limp condition and moved upon each other. They offer, too,
a firm foothold for the thick quills of the big feathers of the wing.
It has been mentioned that all the evidence at our disposal points
to the view that the wing has become gradually moulded into an organ
of flight, from a condition in which it played a different part. The
earliest bird of which we have any record had wings which were much
less perfect as flying organs than those of modern birds. It seems
pretty plain that the bones in that antique bird were much less rigidly
fixed together, and it is equally clear that the fingers were very
much more loosely attached to one another. They were also more on an
equality as regards size; the great disparity evident in fig. 12 is
not to be seen in the Archæopteryx. All this, of course, shows that
the Archæopteryx could not have possessed the ample pinion of its more
vigorous descendants of to-day. The fossil Archæopteryx looks a little
like a crow would look after receiving at close quarters a charge of
duck shot; but a closer examination will show that in reality all the
bones are there, on one side at least. Out of the disjecta membra of
the fossil numerous ‘restorations’ have been put together, which are
as diverse as the minds which imagined them. We cannot really say with
certainty what were the precise relations of the hand to the feathers.
It seems most probable that the hand of this ‘mediæval’ bird still
retained the ordinary functions of a hand; that it served its possessor
to lay hold of convenient branches, from which it fluttered feebly to
others. One bold speculator has insisted upon the probability that the
Archæopteryx had the requisite five fingers of the presumed ancestral
type; but there are no traces of them, except in so far as the lie
of the feathers enables a hint to be gathered. Boring operations, or
at least prospecting in the interior of the stony slab on which the
fossil lies, might reveal some additional fingers; but the operation
would be fraught with too obvious perils to a nearly unique object.
There are a good many birds which do not, and some which cannot, fly.
To the first category belong such birds as the domestic ducks and
fowls, and some of the rails. These birds, when put to it--when chased
by a dog, for example--can often fly; but as a rule they do not, or
at most only flutter along. The Ostrich tribe and a few other birds
have totally lost the power of flight. But though this is the case,
the bony structure of the hand remains the same in the Ostrich and in
the American Rhea; in the Cassowary, however, and the Apteryx of New
Zealand, the fingers are reduced to one. The last stage in the atrophy
of the organ of flight is seen in the giant and extinct birds of New
Zealand, the Moa or Dinornis, in which no trace of a wing has been
so far discovered. But in some of these birds in which the wing is
reduced in size, or so simplified in structure that it can no longer
serve its legitimate purpose, it is made use of for other purposes.
When the Ostrich skims along the surface of the sandy deserts where it
is often found, it holds out both wings, which are compared to sails;
they possibly serve rather as the pole of the tight-rope walker, to
preserve the balance of the bird when hurrying along at full speed. In
the Secretary Vulture of Africa the wings can be used for flying, but
they are also used as weapons wherewith to combat the poisonous snakes
upon which the bird so usefully feeds. It strikes down the venomous
serpent when the latter is attempting to strike the bird. The Chauna
of South America has strong spurs upon its wings, which are used for
fighting as well as for flying. But the most curious use to which
wings are put is afforded by the Penguin. If the reader has never seen
the ‘diving birds’ fed at the Zoological Gardens, let him go there on
the first opportunity, and see how rapidly and gracefully the Penguin
‘flies’ under water by the flapping of its wings. They are shorter than
those of most birds, and the feathers have become flattened and almost
scale-like, so as to offer no resistance to the water; at the same
time the bones of the wing are flattened, so that a broad surface is
provided, which of course acts like an oar. With this oar-like wing the
Penguin can outswim a small fish.
Sternum and Ribs.
The breast-bone or sternum (fig. 8, p. 13) of birds shows the same
relation to the power of flight that is shown by so many, if not
by all, parts of the skeleton. It is relatively a very large bone,
and is in all perfectly flying birds furnished in the middle line,
below, with a strongly marked keel, the presence of which has given
its name to the great group of birds called carinates. The ostrich
tribe, from whose sterna the keel is absent, are termed ‘ratite,’ or
‘raftlike.’ The reason for the keel is the attachment of the great
pectoral muscle, which is the most important muscle of flight. The
sternum often offers useful characters to the systematist. The surface
of the bone is sometimes in various degrees fenestrate, or more or
less deeply incised, the one condition being an exaggeration of the
other, and both the conditions being due to defective ossification.
The sternum is attached to the vertebral column by the ribs, which are
well developed in all birds, but vary very much in number. A highly
characteristic feature of the ribs of birds is a small bony projection
of the hinder margin of a certain number of them, called the uncinate
processes. These are present in all birds, with the single and
remarkable exception of the South American Screamers (_Chauna_,
_Palamedea_), a group of birds occupying a rather isolated
position, and showing resemblances to a great many different groups.
Pelvis.
[Illustration: FIG. 13.--PELVIS AND HIND LIMB OF DIVER.
_c_, _d_, ilium; 63, ischium; 64, pubis; 65, femur; 66, tibia;
67, fibula; 68, tarso-metatarsus; i.-iv. digits with phalanges
numbered.]
The hind limbs are attached to the vertebral column by means of a
considerable bony structure known as the pelvic girdle (fig. 13). This
mass of bone is in reality composed of three pairs of elements, though
they are in the adult strongly compacted together. The main bone, which
is firmly attached to the vertebral column, is the ilium; with this is
almost completely fused the ischium; the very slender pubis is to a
large extent free from these bones. The pelvis is in its form one of
the most characteristic of the bones of the bird’s skeleton. In other
animals the three bones are present, but they are directed away from
each other; in the bird, as already described, the pubis is directed
backwards, parallel to the ischium; in correspondence, perhaps, with
its position it has become a feeble bone, and has but few muscles
attached to it. The interest of the matter, however, is mainly in the
fact that among the extinct Dinosaurs, a race of mesozoic reptiles,
there were some in which the pelvis had a very bird-like structure,
with the same feeble and recurrent pubis. This has been urged as a
mark of affinity between the Dinosaurs and birds. The several bones of
the pelvis are free from each other at the extremity, or almost so,
in all the Ratites, and in the Tinamous, which are supposed to bear
some relationship to the Ratites. The fact is interesting as being an
example of the retention of a character by one group of birds which
is only transitional and embryonic in another, for in all young birds
the bones of the pelvis are separate; it is not until some time before
hatching that they become fused together as we see them in the adult.
Hind Limb.
At first sight there appears to be a considerable difference between
the fore limb and the hind limb. In both there is a long proximal bone,
called humerus in the one case and femur in the other, followed by a
pair of bones--the tibia and fibula--corresponding to the radius and
ulna of the fore limb. But in the hind limb (fig. 13), the foot proper,
consisting of metatarsals and phalanges, appears to come immediately
after the tibia and fibula. In a sufficiently young bird, what is the
apparent lower end of the tibia, and what is equally apparently the
upper end of the metatarsus, are detachable; these two halves which are
thus detachable are the tarsus, which is the equivalent of the carpus
of the wing. The lower bone of the leg is on this account usually
spoken of as the tarso-metatarsus. The lower part of this bone is made
up of three fused elements, the separation of which from each other is
clearly apparent at the lower end of the bone, where the phalanges are
attached. In the Penguins the three bones are separated by grooves of a
very marked character throughout. In some birds there is a fourth toe,
the hallux; in these cases there is a small separate metatarsal loosely
fixed to the lower end of the large conjoint metatarsals.
Gizzard and Alimentary Canal.
[Illustration: FIG. 14.--GIZZARD OF SWAN.
_o_, orifice of duodenum; _a_, end of proventriculus; _cd_,
muscular part of gizzard.]
The gizzard (fig. 14) of the fowl is simply a part of the stomach
which has especially hard and muscular walls, the other half remaining
soft in texture; this latter is termed the proventriculus, and into
it open the mouths of glands which secrete the digestive juice of the
stomach. But the muscular part of the stomach--the gizzard--has to
grind down the frequently hard food of the bird, so it has not merely
a strong wall made of muscle, but also a very tough lining; the whole
organ, therefore, forms a highly efficient mechanism for crushing and
grinding the seeds and other hard vegetable food which is swallowed.
It is rendered more useful still for this purpose by the pebbles which
every bird takes care to swallow. The true and singular stories about
the varied contents of an Ostrich’s stomach are founded upon the fact
that, like other birds, it picks up stones, and with them occasionally
other objects. But all birds do not possess a hard gizzard; in Hawks
and fish-eating birds the walls are thinner, and the organ is flaccid
instead of being rigid. By a very curious and unique exception certain
Tanagers, a race of large, often bright-coloured, American, finch-like
birds, have nothing at all that can be compared to the gizzard of
other birds; this part of the alimentary canal is totally wanting.
Now the difference between the gizzard of the grain-eating fowl and
the flesh-eating hawk is chiefly a matter of diet. The celebrated
anatomist, John Hunter, who lived in the last century, and wrote so
much about the anatomy of all kinds of animals, including birds, found
that he could feed a soft-stomached bird into one with a hard gizzard,
and _vice versâ_.