does not vary a great deal in different birds. The intestines are
always rather short, and are diversely coiled, the method of coiling
being often characteristic of a particular group. A good way down the
intestine are a pair of cæca, which may be entirely absent, as in the
Hornbills, for example; and if present may be extremely short, as in
the Sparrow, or very long, as in the Ostrich; various intermediate
degrees exist. As in all vertebrated animals, two glands pour their
secretion into the intestine; these are the pancreas and the liver.
The secretion of the liver is the bile; this fluid is accumulated as
it is formed in a largish bag--the gall-bladder, in those birds which
possess one. Shakespeare used the epithet ‘pigeon-livered,’ which meant
literally the absence of a gall-bladder; but, oddly enough, there are
some kinds of pigeons which have a gall-bladder, while others, like the
common pigeon, have not. The intestine ends in the cloaca, which is the
common chamber into which the urinary and generative organs also open.
Tongue and Teeth.
In the inside of a bird’s mouth we find only one of the two things that
we might expect to find: there is a tongue, but no teeth. We shall come
back to the teeth immediately. The tongue is not so useful among the
majority of birds as it is in most mammals. But some do make use of
it to a great extent. If you watch a parrot eating its food, you will
observe that its thick and fleshy tongue is of the greatest assistance
in helping it to manipulate the pieces of food--to extract, for
instance, the kernel from a seed or nut. It plays exactly the same part
as it does with us. In one kind of parrot, called the ‘Brush-tongued
Parakeet,’ the tongue is frayed out at the free end into a brush-like
extremity. And there are some small birds, which peck at flowers and
live upon honey, in which the tongue is thin and delicate, and frayed
out in the same way; this allows them to suck up the juices of the
flower. In the Hummingbird the tongue is rolled up so as to form two
tubes running side by side, and the same power of sucking up juices is
acquired by this means, which, curiously enough, is exactly paralleled
by the proboscis of the butterfly. In other birds the tongue is
sometimes merely a thin, flat, horny projection, and in others, again,
it is just not absent altogether.
A little reflection about the habits of birds will show that they
really do not want teeth; and we know that Nature is a most rigid
economist: nothing superfluous is allowed in the body. Even rapacious
birds like Owls and Hawks have no teeth, because they have a powerful
beak and claws, with which the food may be as effectually torn to
pieces. Birds such as the Pigeon, which feed upon grain, possess
a gizzard--which we have had something to say about already--that
performs effectually the function of a mill, grinding into a powder
the hard grains of wheat and other seeds which the bird swallows.
Nevertheless birds once did possess teeth. In earlier times of
the history of this earth there were some birds whose jaws had
as formidable a range of teeth as the mouth of many reptiles.
They were fish-eaters, and have been named _Hesperornis_ and
_Ichthyornis_. The first was something like a Diver in shape,
the latter more like a Gull. A still more ancient bird, the oldest
form of bird known to us, the _Archæopteryx_, had also toothed
jaws. In fact, in the old days it was the rule for birds to have
teeth, whereas now it is the rule, without a single exception, for
birds to be toothless. Perhaps these ancient and extinct forms had
some corresponding disadvantage when compared with their modern
representatives; their teeth and claws, for example, may have been less
effective. But although there is no bird now living which has real
teeth, traces of these organs have been discovered in the young embryos
of certain birds, which seems to be an absolute proof that they, at
any rate, had for their first parents toothed birds. But although
modern birds have no teeth, with enamel, dentine, and so forth, all
complete, the horny beak has occasionally ridges which to some extent
play the part of teeth. The inside of the Duck’s mouth is rough with
such ridges, which occur also in some other birds. The large Flamingo
was for some time regarded as a long-legged and awkward Duck that had
partially adopted the habits of a Stork, partly on account of the fact
that the inner edges of the beak were ridged in a fashion exactly
like that of the Duck. But it happens that there is a Stork, a true
Stork, in India, whose scientific name is _Anastomus_, which has
similar ridges. Ducks feed to some extent upon shellfish, which the
roughened edges of the beak are well suited to crush. The replacement
in the course of ages of true teeth by horny teeth is seen--a curiously
parallel case--in the Duck-billed Platypus of Australia, which has when
adult horny plates instead of teeth, but when young has real teeth.
Heart.
As with all vertebrated animals, birds have a centrally placed heart,
with which are connected arteries and veins, the two systems of tubes
being connected at the ends farthest away from the heart by minute
vessels--the capillaries. In relation, no doubt, to the intelligence
and activity of birds, as compared with their slower relatives, the
reptiles, we find a heart of much more perfect organisation. There
are four distinct chambers, as in the mammal, so that the arterial
and venous blood are separate, and do not commingle. The two sides of
the heart are only in indirect communication by way of the arteries
and veins and capillaries. The left ventricle gives rise to the
aorta, which is the great arterial trunk of the heart; this divides
into the carotid and other arteries, which supply the entire body,
with the exception of the lungs. The blood, which is sent out through
this vessel by the contractions of the ventricle, permeates the
system generally, and is then collected into a series of veins, which
ultimately unite into two great veins, the venæ cavæ in front, and a
large vein situated posteriorly, the inferior vena cava. These pour
the blood back into the right auricle, whence it passes at once to
the right ventricle. From the right ventricle it is driven into the
lungs, whence it is returned to the left auricle, and so into the left
ventricle to renew the circulation. The two chambers of each half of
the heart are guarded from each other by valves, which only allow
the blood to flow in the proper direction, as stated in the above
brief description of the course of the circulation. It is a curious
fact that the valve which separates the right auricle and ventricle
is a completely muscular structure, while the other is membranous.
Moreover, it does not form a complete circle, but is deficient upon
one side of the orifice. The interest of this fact is not merely in
its abnormality, its divergence from what one would expect, but in the
resemblance which is thus shown to a group of mammals, the Monotremata.
This group includes only the Duck-billed Platypus of Australia and the
spiny Anteater (Echidna) of the same continent and New Guinea. In both
of these animals the heart valve in question is also largely muscular,
and does not entirely encircle the opening from the auricle. These two
mammals also, as everyone knows by this time, have the strange habit
for a mammal of laying eggs, which is one among some other reasons
which once led naturalists to place them in the neighbourhood of birds.
The egg-laying, of course, is not distinctive, since reptiles have the
same way of bringing forth their young; and as to the heart valve,
it is rather to be explained by the fact that both types of animals
are low in the scale of their respective groups, and therefore both
approach a common ancestral form.
Voice Organ.
[Illustration: FIG. 15.--SYRINX OF RAVEN (POSTERIOR SURFACE).
_g_, tympaniform membrane.]
[Illustration: FIG. 16.--SYRINX OF RAVEN (LATERAL VIEW).
_a_, _b_, _c_, _e_, _f_, intrinsic muscles; _d_, sterno-tracheal
muscle.]
[Illustration: FIG. 17.--SYRINX OF RAVEN CUT OPEN LONGITUDINALLY.
_i_, pessulus; _h_, vibrating membrane; _g_, membrana
tympaniformis.]
By their voice, too, birds are distinguished from the rest of the
animal creation. Though there may be legends of singing serpents and of
talking monkeys, a harsh scream or a growl is the only manifestation
of the emotions through the voice which exists until we arrive at man.
Among birds, the possession of a melodious voice is limited to that
group which we term the Passeres. Other birds can scream or utter a
dull note, while many are mute. So flexible is the voice organ of these
creatures that they are the only animals that can imitate human speech.
Here, however, it is not only the Passeres which can imitate the
essential attribute of man. The Parrots, of course, are always supposed
to be _the_ birds which can talk, but this is far from being the
truth. The hoarse utterances of most Parrots are left far behind in
clearness of sound and correctness of imitation by the little Indian
Mynah, which may be usually seen at the Zoological Gardens, and heard
to speak. But the Parrot cannot sing. These are the only two groups
of birds which have so elaborate and flexible an organ of voice. From
this it might be inferred that some peculiarities of mechanism would
distinguish the organ in question of these birds, and that is what we
actually find to be the case. But, oddly enough, it is not only those
birds which have a beautiful voice whose voice organs are so elaborate
in structure. The harsh croak of the Raven issues from a syrinx which
is as delicately fashioned as that which allows of the exquisitely
varied tones of the Nightingale. The word ‘syrinx’ has been mentioned;
that is the technical term for the voice organ of the bird, which is
formed from a part of the windpipe, as in man and the mammalia, but
from a different part of that tube. In man and in mammals the voice
organ is placed in the throat just a little way down, at the prominence
often spoken of as ‘Adam’s apple.’ This is a wider part of the tube,
with larger rings of cartilage, which contains a pair of tightly
stretched membranes that can be made to vibrate and cause a sound. In
the bird, the voice organ is situated farther down, at the very point
where the trachea forks into the two bronchi, one for each lung. Here
are figures which illustrate the voice organ of a singing-bird (figs.
15, 16, 17). At this forking of the trachea the rings of the tube,
which are of gristle or cartilage, become somewhat different in form.
In the middle is a piece, which is often converted into bone, like the
‘three-way’ piece used to fix together the stick and the hoop of cane
of a butterfly-net. To the upper side of this, and therefore within
the tube, and directed upwards, is a little crescent-shaped piece of
membrane (_h_, fig. 17); this can be set vibrating by the stream
of air passing up and down the windpipe. At the sides of the syrinx
there are shown in the figure (fig. 16) three pairs of muscles;
these when they contract shorten the syrinx, and of course produce
alterations in the note, just as the shortening of the tube in a cornet
alters the sound. In many passerine birds, and in most other birds,
there is only one pair of these muscles; but the Parrots agree with the
passerines in having several pairs of muscles, and therefore a more
easily alterable syrinx. In a good many birds there are no muscles at
all in this place; for example, in the Storks, which have not by any
means a flexible voice. The syrinx, in fact, is one of those organs
which show a great deal of difference in different kinds of birds.
But it is never entirely absent, though rather rudimentary in the
Ostrich. The Australian Emu has a curious way of producing its sounds
which is not found in any other bird. The cock and hen Emus can only
be recognised by their voice, which is duller in the hen and sharper
in the cock. When the bird is uttering its note, it seems almost to
come from somewhere else, and not from the throat of the bird; the bird
is something of a ventriloquist. The sound, which is a low bellow, is
produced by a bag of skin opening into the windpipe some way up the
neck; a current of air passing down the tube is believed to set the air
in this bag in vibration, just as the air in a key may be caused to
vibrate by blowing over its edge. Generally speaking, the windpipes of
birds are straight tubes running to the lungs by the shortest route;
but in the Cranes, and in a few other birds, the pipe is coiled upon
itself once or twice, and the coils are even hidden in an excavation of
the breast-bone. The increased length of tube gives a louder and more
resonant note, such as we know characterises the Crane.
Lungs and Air-sacs.
It is not only by virtue of their powerful muscles and stiffened fore
limbs that birds can fly. The body is rendered lighter in proportion
to its bulk by air-cavities, which permeate everywhere, even into the
substance of the bones. So thorough is this aëration in the Screamer
of South America, that when the skin of the recently dead bird is
roughly pressed it crackles. Curiously enough, there seems to be no
very definite relation between the degree of thoroughness to which the
aëration of the body is carried out and the capacity for flight. The
Screamer, that has just been mentioned, is fuller of air-cavities than
the Frigate-bird, in which the art of flying is carried to the highest
extreme--the ‘triumph of the wing,’ as Michelet says in ‘L’Oiseau.’
Anyone who has the opportunity of dissecting a Hornbill will be struck
by the large and abundant air-spaces between the muscles. This applies
even to the Ground Hornbill of Abyssinia; and yet the latter, as
its name denotes, lives upon the ground, while the flight of other
hornbills is heavy and most unsuggestive of lightness of body. These
air-spaces are in direct communication with the windpipe. It is much
easier to understand their arrangement by the actual dissection of a
bird. We must first get a notion of the position and form of the lungs,
which differ very much from the lungs of other animals. In a rabbit,
for example, or any other mammal, the lungs lie freely on each side of
the heart, and are capable of being pushed here and there after the
body is opened, and of much expansion and diminution of volume during
the movements of respiration. But the lungs of all birds are tightly
fixed to the wall of the chest cavity, being, as it were, moulded on to
the ribs and vertebræ; when they are carefully picked away from their
place, they retain the impressions of the bones which they touch. There
is no great possibility here of independent movements on the part of
the lungs. Respiration is effected in a totally different manner; it
is, in fact, bound up with the mechanical filling of the air-spaces.
Each of the two lungs is contained within a large compartment, which is
bounded externally by an obliquely disposed septum, often spoken of, on
account of its direction, as the ‘oblique septum.’ Others call it the
diaphragm, imagining that it is the equivalent of the diaphragm in the
mammal, that partly fleshy, partly tendinous plate which shuts off the
cavity of the chest, in which lie the heart and lungs, from the cavity
of the abdomen, in which lie the intestines, stomach, and liver. Now,
this oblique septum does not by any means closely invest the lungs; on
the contrary, a deep space is thereby shut off, at the bottom of which
are the lungs. This cavity is subdivided by two partitions into three
separate compartments. It requires a very skilful manipulation to show
the fact, but it can, with care, be demonstrated that each of these
compartments is lined by a delicate membrane, which is continuous with
the lung, and is actually a kind of bubble, as it were, blown out of
the lung; these delicate sacs are the air-sacs. There are altogether
nine of them, but all these sacs do not lie within the cavity bounded
by the oblique septa. The largest pair of all the abdominal air-sacs
project into the body cavity far behind the gizzard. Now these sacs
are fairly easy to see in a dissection; but it is not so easy to make
out that they are all of them, except the middle two, connected with
a system of ramified air-spaces which, as already said, permeates the
body generally, lying among the viscera, between the muscles below
the skin, and deep into the actual interior of the bones. But though
it is difficult to see this by a dissection, it is easy enough to
prove it by inflating them. If a syringe is passed down the windpipe
and tied carefully into it, so that no air can escape at the sides,
and air is blown down the tube, the passage of the air into the skin
and other parts can be followed; if a bone be cut across, the air can
be noticed to issue from the cut surface; and if the experiment be
varied by using a coloured fluid instead of air--which is pumped in by
a syringe--the fluid can be seen to ooze from the end of any bone or
muscle that has been cut across. A bird, therefore, when it takes in a
deep breath, not only supplies its lungs with fresh air, but fills its
whole body with the superfluous air. It has been proved that a bird can
continue to breathe if it be held under water, and only the end of a
broken limb allowed above the surface; for, as all the spaces of air
are in communication with the lungs, they (the lungs) can obviously
be as conveniently filled from one end as from the other. When you
are bathing, and take a very deep breath as you are swimming, you can
detect a sensible increase in the buoyancy of the body; in a bird, of
course, the difference is enormous, after the sacs are filled, from a
condition of comparative emptiness. The way in which a bird breathes
is different from the way in which a human being breathes. There is,
of course, the essential resemblance that is shown between all animals
that have definite organs which are set apart for respiration: the
feathery gills of the marine worms, the closely set branchiæ of the
fish, the lungs of the bird and of the mammal, are all constructed
upon one plan, so far as essentials are concerned. In all of them
blood-vessels are brought into close relation, though not into actual
contact, with water or air containing oxygen. The blood-vessels are
separated from the water or air by the thin membranes of the lungs
or gills, through which the oxygen can pass in to the blood, and the
carbonic acid and effete gases can pass out; it is this exchange which
is the essential act of respiration. We cannot, however, in this book
pretend to go into general matters of this kind, which would take us
too far from the subject at hand; but anyone who would pursue this
further can consult Professor Huxley’s ‘Elementary Physiology,’ or any
other elementary text-book upon physiology. When a mammal--a human
being, for example--breathes certain muscles are called into play. If
a person is watched, it will be seen that the chest expands during
inspiration, and that its calibre diminishes during expiration. What
happens is this. The lungs are contained in a cavity which contains no
air. This cavity can be increased in size in two directions. When the
ribs are moved out--which they can be by the movements of the muscles
called intercostal, which lie between them--the cavity of the chest
from before backwards is evidently enlarged. On the other hand there
is the diaphragm, which we have already spoken of as bounding the
chest cavity below. Now this diaphragm is muscular, with a tendinous
centre. When the muscles contract, like all muscles do, the surface
of the diaphragm, which was before rather convex towards the chest
cavity, becomes more flat; hence the cavity lying above it, _i.e._
the chest cavity, becomes larger in a downward direction also. When
it is increased in this way by the action of the two separate sets of
muscles, some space--more space than before--is left between its walls
and the lungs which lie within it; it follows, therefore, that, as
there is no air in the cavity, the pressure of air outside the body
forces more air into the lungs, because there is no counterbalancing
pressure to prevent this. The principle is the same in the bird,
but the details are different. If you will turn again to the bird’s
skeleton, you will see that the backbone and ribs and sternum form a
bony box, which is jointed in the middle; this acts precisely like a
pair of bellows: the bones at top and bottom represent the wood, and
the soft intervening leather of the bellows is represented by the
muscles which lie between, and which connect the sternum with the
abdomen and with the ribs. When these muscles contract, the sternum
is obviously brought nearer to the backbone, and air is expelled
from the inside; when they are relaxed, a vacuum is created and air
rushes in. The air-spaces, then, are really ramified tags of lung
which have no blood-vessels in their walls, and are therefore not
meant for respiration, but serve as reservoirs of air, lightening
the body of the creature. It is curious that birds are not the only
animals which possess expansions of lung that are apparently useless
for breathing purposes. The lungs of the Chameleon have quite similar
sacs appended to them. There is, it is true, no such complicated a
ramification as that which we find in the bird, but still there is no
doubt that the structure is of the same nature. It looks almost like
a first step in the path towards a bird. Very possibly the extinct
Pterodactyles, which flew through the woods of the middle ages of the
earth, had bodies lightened in the same or a similar way; for we know
that their bones have thin walls, the large cavity of which in all
probability contained air-sacs. Even some of the jumping Dinosaurs, to
which reference has already been made, seem to have possibly had lungs
constructed on the bird type. We see, therefore, that even where a bird
is, so to speak, most characteristically a bird--in the subsidiary
mechanisms of flight--it betrays a likeness to the comparatively
grovelling reptile, letting alone the aërial and more bird-like
Pterodactyles.
Brain.
The brain of birds is large in proportion to the body, thus contrasting
with that of the unintelligent reptile. From some tables on the matter
which have been published, it appears that, if weight of brain goes
for anything, the goldfinch is one of the most intelligent of birds.
The weight of its brain is one-fourteenth of the entire weight of the
body. The most unintelligent of all is the domestic fowl, whose body is
412 times heavier than its brain. The size of brain, however, seems to
be largely a matter of the size of the bird: generally speaking, the
smaller birds have heavier brains, and _vice versâ_. One might
have expected something from the apparently intelligent Parrot; but the
brain of the ‘Amazon’ is only one forty-second part of the weight of
its body. Even the cruel and bloodthirsty Hawk, which one associates
with brutality and ignorance, has a brain which is but little heavier.
The front part of the organ, known as the cerebral hemispheres, or,
more briefly, as the cerebrum, is that part of the brain which is
associated with intelligence. Now among the mammals this part of the
brain is generally much furrowed, the brain surface being, therefore,
increased without any actual increase in the skull-space required. This
furrowing is met with in most mammals, but not always in the smaller
and in the less intelligent kinds. But in the bird’s brain there are
no convolutions: the surface is as smooth as in the reptile. Not even
in the artful Raven, which some hold as the most highly developed of
birds, is there a trace of the furrowing which one rightly associates,
so far as the mammalia are concerned, with a high position in the
series. The hinder part of the brain is known as the cerebellum;
between this and the cerebrum are the optic lobes, of which there are
only two, the mammals having four. From the brain arises the spinal
cord, or marrow, which runs in the canal formed by the vertebræ, just
as the brain lies in the brain-case. The nerves of the body come
off either from the brain or the marrow, but it is not important to
enumerate them. They show no difference in different kinds of birds.
The Muscles.