THE ANATOMY OF A BIRD.[138]
The Three Divisions of the Class Aves--ANATOMY OF A
BIRD--The Skeleton--Distinctive Features--Peculiar Bone
Character--The Skull--Difference between the Skull of
Birds and that of Mammals--The Jawbones--Vertebral
Column--Sternum--Fore-limbs--Hind-limbs--Toes--The
Muscular System--How a Bird remains Fixed when Asleep--The
Oil-gland--The Nervous System--The Brain--The Eye--The
Ear--The Digestive System--The Dental papillæ--The
Beak--Tongue--Gullet--Crop--Stomach--Uses of the
Gizzard--Intestine--The Liver, Pancreas, and Spleen--The Blood
and Circulatory System--Temperature of Blood of a Bird--Blood
Corpuscles--The Heart--The Respiratory System--Lungs--Air-sacs--The
Organs of Voice--The Egg--Classification of the Class Aves.
Birds may be separated into three great divisions: the _Carinatæ_, or
birds with a keeled sternum, the _Ratitæ_, or birds having a raft-like
sternum, and the _Saururæ_, or lizard-like birds. The last of these
orders links the birds with the reptiles, and does not concern us here,
as it contains only one genus, and that a fossil one, the _Archæopteryx
lithographica_, respecting which a few words will be found at the end
of this article (Vol. IV., pp. 236-8). The other two divisions are of
great importance, and are easily recognisable, although the characters
which separate them are chiefly anatomical. The principal point of
difference lies in the sternum, or breast-bone, and the name _Carinatæ_
is given to all those which have a keel (_carina_) or sternal ridge
largely developed, as in the common fowl; and this is present in the
great majority of birds. The _Ratitæ_ have not got this keel, and in
this division are found the Struthious birds--Ostriches, Cassowaries,
&c. They are all species which cannot fly; and although the number at
present existing is small, the fact of their being found at widely
distant parts of the earth--in South America, in Africa, and again in
Australia--would seem to indicate that they were once more plentifully
distributed, and that they are remains of what was formerly a large
and important group. To these Ratite birds belonged also the extinct
gigantic Moas of New Zealand, and the _Æpyornis_ of Madagascar.
Before proceeding further, it will be necessary to give a brief outline
of the principal anatomical features of a bird’s body. On examining
either the general features of the skeleton, or the minute characters
of many of the bones of which it is made up, in such a bird as a Hawk,
for example, we are arrested by those remarkable arrangements by which
this part of the body is adapted to the mode of life of its possessor.
Here, however, as in so many instances, we have to distinguish between
what is characteristic of the bird as a flying animal, and what is
more or less common to it and other vertebrate animals, and does not
especially relate to peculiar habits. We may well be struck by the
marvellous power which birds have, and which man has not, and it is
curious to notice how man’s aspirations have ever been associated with
it. Without pausing to observe that painters always endow spirits with
wings, or that the imaginative genius of the French has emboldened them
to form a “Société d’Encouragement pour la Locomotion Aérienne,” we may
find in the words of Faust definite expression of what man feels with
regard to the law by which he is held down to earth:--
“Ich eile fort, ihr ew’ges Licht zu trinken,
Vor mir den Tag und hinter mir die Nacht,
Den Himmel über mir, und unter mir die Wellen.
Ein schöner Traum, indessen sie entweicht.
Ach! zu des Geistes Flügeln wird so leicht
Kein körperlicher Flügel sich gesellen.
Doch ist es jedem eingeboren,
Dass sein Gefühl hinauf und vorwärts dringt,
Wenn über uns, im blauen Raum verloren,
Ihr schmetternd Lied die Lerche singt,
Wenn über schroffen Fichtenhöhen
Der Adler ausgebreitet schwebt,
Und über Flächen, über Seen
Der Kranich nach der Heimat strebt.”[139]
Inspired by feelings such as those so powerfully expressed in Goethe’s
lines, numerous naturalists have treated of the bird as though the
powers of flight were confined to it, and were not shared by Bees and
Bats in the present, and by Pterodactyles in the past. With this word
of comment, which it is even still necessary to insist upon, attention
should be given to the following avian characters:--The anterior limbs
do not touch the ground, and the bones which compose them are adapted
for carrying the feathers of the wing; the breast-bone is not only
elongated, but has its central portion developed (except in the Ratitæ)
into a strong keel, the better to permit insertion of the muscles
by which the fore-limb is moved; the small bones (vertebræ) in the
region of the back are fixed firmly together, and are not, as in man
or in the Ostrich, movable on one another; while those which succeed
them are often welded into one mass with the greatly-developed upper
bones (_ilia_) of the pelvic girdle; and the hinder vertebræ develop
an upstanding plate (ploughshare bone) which gives support to the
_rectrices_, or so-called steering feathers of the tail. It will have
been seen that the ordinary seizing organ of man (the hand) has in
birds been modified to serve another purpose; but this is made up for,
not only by the character of the beak, but by the long and flexible
neck, and in some by the grasping toes.
Before describing in detail the characters of the different parts of
the skeleton, it is to be noted that many of the bones are not, as in
the Mammalia, filled with marrow, but with air; a large cavity may,
for example, be seen in the upper bone (_humerus_) of the wing of the
common fowl. It is obvious that the specific gravity, or weight of the
bird, is thus largely reduced, while the connection between these
air-spaces and those which are derived from the lungs enables the
contained air to undergo the necessary exchanges with the surrounding
medium.
It was long ago observed by the famous German anatomist, Johannes
Müller, that “it has often been a subject for complaint that the
anatomical characters of birds are so constant that they are of but
slight assistance in the labours of the zoologist.” The truth of this
will very forcibly strike any one who comes to the study of the skulls
of birds, after having examined a series of skulls in mammals, so that
the seemingly trivial variations to which anatomists have directed
attention are in truth those which are, in birds, often of the most
importance.
[Illustration: SKELETON OF EAGLE. (_Reduced. After
Milne-Edwards._)
(_p_) Pelvis; (_c_) Coracoid; (_dr_) Dorsal ribs; (_sr_) Sternal
ribs; (_up_) Uncinate processes; (_co_) Coccyx; (_r_) Radius; (_u_)
Ulna; (_d_) first phalanx of chief digit of the wing; (_d′_) second
phalanx of chief digit of the wing; (_d″_) Phalanges of lower digit;
(_d‴_) Pollex; (_ca_) Carpus; (_f_) Femur; (_fu_) Furcula; (_h_)
Humerus; (_pp_) Postorbital process; (_tm_) Tarso-metatarsus; (_m_)
Metatarsus; (_ma_) Lower jaw; (_me_) Metacarpus; (_s_) Scapula;
(_pa_) Phalanges of foot; (_fi_) Fibula; (_pt_) Patella; (_st_)
Sternum; (_ti_) Tibia.
]
The skull, then, is, as compared with the rest of the body, small; but
that portion which contains the brain is relatively larger to the face
than it is in any living mammal. The orbits, or cavities in which lie
the eyes, are very deep, in consequence of the small extent to which
the walls of the brain-case extend forwards. The cavities of each side
are separated by a partition (_inter-orbital septum_), which is more
or less bony; the nasal bones are short, so that the nasal orifices
(anterior _nares_) are placed near to where the beak joins the face. Of
the four bones which bound the great opening at the back of the skull
for the passage of the spinal cord, three take part in the formation
of the single ball-like projection, or condyle, by which the skull
is hinged on the vertebral column. In this point, the skull of birds
offers a striking point of dissimilarity to that of mammals, in which
there are two condyles, one on each side of the great opening (_of_).
Another point in which birds do not resemble mammals is in the mode
by which the lower jaw is hinged on the skull. This is in the case of
birds effected by a bone, which, being more or less square in shape,
has gained the name of the _quadrate_ (_q_). In mammals the skull
proper and the lower jaw are directly connected. This quadrate bone is
connected by a long narrow bar (_quadrato-jugal_) with the bones which
go to form the “beak,” and also, by a narrow bone directed inwards,
with the bones which lie in the middle line of the base of the skull,
and form the hard palate. The connections between these bones are often
of such a kind as to allow of the upper jaw, or upper half of the beak,
being movable on the rest of the skull, the upper bones of which are so
completely united together as to form a very firm point of support. In
the Parrots this arrangement is carried to an extreme, for the slender
bones (nasals and processes of the pre-maxillaries) which connect the
upper jaw with the bones of the brain-case form with them a distinct
joint, and so allow of that large amount of vertical movement which
will have been observed in these birds. The pre-maxillary bones (_pm_),
which are so small in mammals, are very largely developed in birds,
giving off, as they do, three processes: one to the _frontal bone_ (or
fore-bone of the brain-case), one along the hard palate, and another
externally to form the margin of the beak. The parts that vary most in
this bone-group are the bones which make up the hard palate. Of these,
the chief are the so-called _palatines_ (_p_) and the _maxillaries_;
the former are united by an articular surface with the bone which forms
the anterior part of the base of the brain-case, while there is in the
middle a narrow bone, which, from its shape in man, has received the
name of the _vomer_ (ploughshare, _v_). The maxillary bones develop
horizontal plates, which have the palate below and the nasal chamber
above them.[140]
The lower jaw (_ma_) is composed of six pieces of bone on each
side--the dentary, angular, surangular, coronoid, splenial, and
articular. The upper part of the joint is concave.
The tongue is in relation, as regards its support and movements, to
the _hyoid_ bones, which will be especially noticed in describing the
Woodpecker.
[Illustration: SKULL OF YOUNG OSTRICH FROM ABOVE (A) AND FROM BELOW (B).
(_After Owen._)
(_of_) Occipital foramen; (_so_) Supraoccipital; (_eo_) Exoccipital;
(_q_) Quadrate; (_pa_) Parietal; (_pp_) Pterygoid process; (_f_)
Frontal; (_e_) Ethmoid; (_n_) Nasal; (_pm_) Premaxillary; (_m_)
Malar; (_p_) Palatine; (_v_) Vomer; (_im_) Intermaxillary; (_l_)
Lachrymal bones.
]
Turning to the vertebral column, we find a number of small bones,
complicated in form, and more or less movable on one another. For
convenience of description they may be divided into those which belong
to the neck (_cervical vertebræ_), to the trunk (_dorsal vertebræ_), to
the _sacrum_ (so-called because it was offered in sacrifices!), or to
the tail (_caudal vertebræ_). As has been observed already, the first
of these, or the region of the neck, is very long, and is always long
enough for the beak to be able to reach to the base of the tail. In
birds, unlike mammals, the number of these cervical vertebræ may be
as low as nine, or as high as twenty-four. The first of them, which
is known as the _atlas_, has on its front face a rounded cavity into
which fits the single projecting condyle, which was spoken of as being
found at the back of the skull; and this condyle, being well rounded,
is easily able to turn in the cavity which it fits, and the head is
thereby capable of a large amount of movement. In the succeeding
vertebræ it is possible to make out a body, an upper arch, through
which passes the spinal cord, which meets above in the middle line, and
is produced into a more or less long spinous process set horizontally
to the “body,” and others directed forwards and backwards, so as to
connect each vertebra with its neighbours; and lastly, a lower arch,
the two halves of which are not connected below, but are converted
into the more or less long ribs. As these vertebræ are so small it is
clear that if their spines were long the free movement of the neck
would be greatly impeded, and they are therefore in many cases little
more than projecting processes. This free movement is further greatly
aided by the characters of the two faces of the body (or _centrum_)
of each vertebra; the face of each is saddle-shaped, that is to say,
the anterior face is concave from side to side, and convex from above
downwards, while the reverse of this is seen on the posterior face; in
addition to this the vertebræ are separated by a disc of cartilage from
one another. The region of the neck is, broadly speaking, distinguished
from that which succeeds it by the fact that the ribs connected with
its vertebræ do not reach to the sternum, or breast-bone. In all birds
which are capable of flight this dorsal region has its parts firmly
united together, and the same holds for the parts which follow, till
we reach the region of the tail, where the more anterior vertebræ are
movable on one another, so as, perhaps, to serve in aid of the steering
organ formed by the rectrices, or feathers (_co_). In all living birds
the caudal vertebræ are a good deal shorter than the body, but in the
fossil _Archæopteryx_ they are longer.
The only important point to note with regard to the ribs, is the
presence on some of them of backwardly directed hooked processes (_up_,
fig. on p. 241), which aid in giving firmness to the thoracic region.
The number of ribs is variable, but there is never a large number
connected with the dorsal vertebræ, as there are in some Carnivora, in
Hyrax, and in the Horse.
[Illustration: STERNUM OF FREGILUPUS VARIUS. (_After Murie._)
(_cl_) Clavicle; (_sc_) Scapula; (_co_) Coracoid; (_cs_) Keel of
Sternum.
]
The fore and hind limbs are connected to the body by a series of
bones, which form the breast and hip girdles respectively; with
the former series is also connected that large, long bone with its
sharply-projecting ridge (_is_), which is known as the breast-bone, or
sternum, and in the depressions on which so much muscle is collected.
This sharply-projecting ridge to the sternum, which is known as the
carina, or “keel” (_cs_), is found only in the flying birds, though
here and there, as in the Parrot of New Zealand (_Strigops_), it is
very rudimentary. The lower edge of the bone is often imperfect, so
that, as in the fowl, there are two deep clefts on each side, or
there may be but a single cleft, and this again may be converted into
a rounded space; in all cases these clefts or holes are covered, or
filled by membrane, during the life of the animal. Projecting in front
of the sternum, and often intimately connected with it, are the two
clavicles (_cl_), which unite in the middle line to form the bone of
childhood’s delight--the furcula, or “merrythought.” Above, this bone
is connected with two bones, one of which, called the coracoid (_c_),
descends on each side to fit into a depression on the upper edge of
the sternum, while the other, known as the scapula, or shoulder-blade
(_sc_), is set at an angle to the coracoid. The scapula has a backward
and downward direction; while it may be noted that among mammals the
coracoid is well developed only in _Echidna_ and _Ornithorhynchus_.
These two last bones form, at their point of junction, a cavity into
which is fitted the head of the long bone of the arm (wing). In the
Ratitæ, it must be observed, these two bones are not set at an angle to
one another, and they become more firmly united together.
[Illustration: PELVIS OF AN ADULT FOWL, SIDE VIEW. (_Reduced._)
(_After W. K. Parker._)
(_il_) Ilium; (_is_) Ischium; (_pb_) Pubes; (_dl_) Dorso-lumbar
vertebræ; (_cd_) Caudal vertebræ; (_am_) Acetabulum.
]
As in all the vertebrate animals except fishes, the fore-limb may be
divided into three parts (fig. on p. 237)--upper arm, in which there
is one bone, the humerus (_a_); fore arm, in which there are two,
radius (_d_) and ulna (_c_); and hand (E), which can again be
divided into three parts, which in man would be called wrist, palm,
and fingers. Now, in some animals the wrist-bones may be ten in
number, and the palm-bones five, while the number of small bones in
the fingers varies a good deal, but the number of fingers is _five_.
In most birds all these numbers are reduced. Just beyond the fore
arm, the larger bone of which has often small projections indicating
the points at which the secondary feathers have been attached, there
are two small bones (_f_), then comes a longer bone (_g h i_), as it
seems, in which there is an elongated space. Now, this bone consists
of three metacarpals and one wrist-bone; the two outer metacarpals are
absent, the two innermost ones have completely united with one another,
and with the (true) middle metacarpal bone at their upper end; while
the second and third metacarpals are also united at the other--or
finger--end. The inner digit (_k_), or that which corresponds to man’s
thumb, has two joints (_phalanges_), and may be clawed; the next has
three joints, and may also be clawed; while the third finger, which
has never more than two joints, is never known to carry a claw. In
the _Archæopteryx_ the metacarpal bones are well developed, and are
not, as in recent birds, united together. No idea of a bird’s flying
powers can be fairly gathered from the length of the hand, for it is
long in Swifts and short in Albatrosses, for example; although it is to
be noted that in the former the single bone (_humerus_) of the arm is
short, and in the latter long.
As in the breast-girdle, the bones of the hinder or hip-girdle, by
which the hind-limbs are connected with the body, are three in number;
of these the upper one is greatly flattened out and projects very far
forwards, thus aiding in the formation of the firm back of flying
birds; the other two bones are much more slender, and are directed
backwards and downwards. It is a curious circumstance that it is in
one bird only, in either case, that these bones are directly connected
at their lower ends with their fellow on the opposite side; those
which are known as the _pubes_ (_pb_) are so in the African Ostrich,
and those which are known as the _ischia_ (_is_) in the Rhea of South
America. These two bones, with the large, flat _ilia_ (_il_), take
part in forming the cavity in which the head of the thigh-bone plays;
the outer of the two bones (_fi_) which are found in the leg is rarely
as long as, and is always much more slender than the other (_ti_),
which has a strong ridge on its front face. There is yet another very
remarkable point of resemblance between birds and reptiles, in that
the “ankle-joint” is in both cases situated between the two rows of
bones which make up the “ankle” (_tarsus_). In birds this arrangement
is carried to a still further extent, for the single bone of the upper
row is early united with the shin-bone, as may be seen under those
unfortunate circumstances in which the poulterer has provided an aged
fowl (aged, that is, for eating); in more fortunate cases it will be
found possible to separate a small bone from the lower end of the
shin-bone of the leg.
In no case does any bird, even _Archæopteryx_, possess a fifth toe.
Unlike mammals, the number of joints in the toes varies greatly in
birds. In those which possess four toes we find the following number of
joints: in the first, two; in the second, three; in the third, four;
and in the fourth, five. This rule holds for nearly all birds, but the
Swifts have never more than three joints, and in the Goat-sucker and
the Sand Grouse there are two less than ordinary on the fourth toe. In
a number of birds the inner toe (big toe of man) disappears, and in the
Ostrich proper the next division of the “typically” five-toed foot, or
second toe, has no toe-joints.
In dealing with the muscular system of birds, we need here concern
ourselves with only those special muscles which are modified in
accordance with the necessities of the bird’s habits, and those other
muscles which have been brought into special notice by valuable
investigations.
That great fleshy mass which is found on the breast of a bird, and
which is not unknown to those who are fond of a good “dish,” consists
of three separate muscles, two of which depress, while the other
elevates the wing. The presence of the elevator muscle on the lower
side of the sternum is a curious arrangement by which the centre of
gravity of the animal is lowered--a most necessary condition in flight;
the tendon from this muscle passes through a pulley-like canal to
be inserted into the upper side of the head of the bone, which, as
has already been explained, is known as the humerus, so that when it
contracts it draws this bone up. The ability of the wings to resist
the pressure of the air is clearly dependent on the power possessed by
these muscles. Borelli has calculated that the “pectoral muscles” of
the bird exceed in weight all the other muscles taken together, whilst
in man the pectoral muscles are but a seventieth part of the mass of
the muscles.
The large and important muscles, which in the Mammalia, constitute the
_diaphragm_, or midriff, are ordinarily said to be absent in birds,
and, indeed, in most cases are but feebly represented. In the Ratitæ,
and especially in the New Zealand form (_Apteryx_) of this group, the
diaphragm may attain to a very fair degree of completeness, though
even here the apex of the heart is allowed to pass into the abdominal
cavity. The muscles of the back are feebly developed, as might be
imagined from the firm character of the spinal column; and as the fore
limb exhibits but slight power of _varying_ its movements, its muscles
are not well developed. Those muscles which are found in the skin are,
on the contrary, expanded into broad pieces; and special bundles are
sent to the larger feathers of the wings and of the tail, and to those
folds of skin which connect the upper arm with the trunk, and with
the fore arm, respectively. Borelli thus explains the arrangement by
which a perching bird remains fixed when asleep: A muscle which arises
from the _pubes_ bone of the hip-girdle passes over the knee, and then
takes a backward direction so as to pass behind the ankle; it thus
becomes one of the flexor muscles, by the contraction of which the
toes are flexed, or bent. When the perching bird, which, as we know,
has one of its toes directed _backwards_, is seated on a bough, the
thigh has its upper end directed backwards, while the upper joint of
the leg is turned forwards, or in other words, the two parts of the
leg have opposite directions. This arrangement acts as a contracting
influence on this muscle and its tendons, while the weight of the bird
is sufficient to preserve this condition and the consequent flexion of
the toes.
To turn to those muscles the arrangement of which has, been made the
basis of a suggested classification. In the leg of the bird there are,
among others, four muscles, the names of which are _femoro-caudal_,
_accessory femoro-caudal_, _semi-tendinosus_, and _accessory
semi-tendinosus_, any of which may be absent, but in those cases where
a single muscle only is found the first is always present; again, there
is a muscle which, from its course, is known as the _ambiens_, and
this, too, may be present or may be absent. As the presence or absence
of any of these muscles is a very constant phenomenon in any given
section of birds, it has been proposed to divide the class into those
which do, and those which do not, possess the above-named _ambiens_
muscle. In the latter group the second of the four above-named
muscles--the accessory femoro-caudal--is never present.[141]
Of all the muscles, those which act in aid of the vocal organs are of
the greatest interest, but they will be considered a little later on.
A valuable suggestion has been made, which, if followed out, may lead
us to understand how it is that the brain of the bird, which is so
simple as compared with that of man, is nevertheless capable of so much
intelligent activity. Bearing in mind the axiom that it is quality not
quantity that tells, and looking at the fact that the brain of the most
highly intelligent man is, after death, supposed to be similar to that
of the foolish and of the unwise of our race, it is obvious that the
essential difference must lie elsewhere than in the coarser, or more
evident, characters of that organ which is known as the brain. The
suggestion, then, that was made, was to the effect, that the possessors
of aviaries, in which it was possible to study the characters of birds,
should submit the brains of their deceased favourites to that more
thorough investigation which the microscope allows of. The brains of
birds vary but little in their anatomy. The optic lobes are rounded,
paired, and tubercular in the bird, and are not divided into four, as
in mammals; they are found at the lower part and sides, and not in the
upper part of the brain. The _cerebellum_ is not continued at the sides
into distinct lobes; nor are the two lobes of the brain (or _cerebral
hemispheres_) provided with those convolutions which, in mammals, seem
to increase in complexity of character as the animal rises in the scale
of intelligence. The cerebrum does not cover the cerebellum. Small as
is the brain of birds, it is found that, in many, its weight is, as
compared with that of the body, much greater than it is in man.
With reference to the spinal cord, or the continuation of the central
part of the nervous system through the vertebral column, it is
only necessary to remark that it is much increased in width at the
two regions, in which the nerves for the fore and hind limbs are
respectively given off; that there is a narrow canal running along its
centre, and that at the lower end there is a large space. In regard to
the cerebral nerves, those for the eyes are of great size.
Coming now to consider the organs of the senses, and beginning with
the eye, it is interesting to note that there are no blind birds, and,
indeed, the eyes are of a large size as compared with the brain. They
are generally placed at the sides, though the nocturnal birds of prey
(in which they are directed forwards) are an exception to this rule.
It is in very rare cases that eyelashes are present, and although they
seem to exist in the group just mentioned, it is probably more correct
to look upon them as slightly modified feathers.
If the eye be regarded as having on its front face, a part which would,
if completed, form part of a smaller circle than the rest of the eye,
it is clear that this _cornea_, or front part, would be more convex
than the rest, and that it would consequently be a “more powerful
glass,” inasmuch as it would exert a greater bending (refracting)
influence on the rays of light which pass through it, while, further,
it is clear that the more convex it is the better “glass” would it be.
Now this is just what happens in birds: the _cornea_ is very convex;
in addition to this, the long axis of the eye, on the length of which
it seems that, in many cases, the condition known as that of being
“short-sighted” depends, is very long in some birds, and notably in the
Owls.
The eye is covered in by a firm and strong membrane, which is known
as the “sclerotic;” this, in its front part, develops a number of
bony plates; of these there may be as many as twenty, and they are
capable of a certain amount of free movement on one another. What
is known as the power of accommodation depends upon the extent to
which the front face of the somewhat lens-shaped body which helps to
separate the eye into two chambers is capable of being rendered more
or less flat; this front face is covered by a membrane which is found
to be more or less taut, according to the state of contraction of the
muscles (ciliary muscles) connected with it. A very little reflection
is sufficient to show that a swiftly moving animal has the focus of
its eye, or the point at which clear vision is alone possible, changed
much more rapidly than an animal which moves more slowly. So much on
the one side. On the other, it is to be observed that muscles vary
in structure; they are either “smooth” or “striated,” and it is the
latter that contract the more rapidly. Putting these two series of
observations together, it is easy to arrive at the result that a bird
should have striated muscular fibre in its ciliary muscles, and a more
slowly moving animal like man, smooth muscular fibres; and this we find
to be the case! The _iris_ is an arrangement by which the quantity of
light admitted into the eye is enabled to be varied, and the small hole
in the centre, through which the rays of light pass, is known as the
_pupil_; this is always rounded in birds, and is never elongated as it
is in some mammals--the Cats, for example.[142]
But the most peculiar arrangement in the bird’s eye is the presence,
projecting into the hinder chamber, of a membrane in which run
blood-vessels; this, which is known as the _pecten_ (comb), or
_marsupium_ (pouch), enters the vitreous humour, which fills up this
hinder chamber by the same cleft as the optic nerve. It is folded, and
is generally of a quadrangular shape; it is not found in the eye of the
Wingless Bird of New Zealand (_Apteryx_).
[Illustration: SECTION OF THE EYE OF THE COMMON BUZZARD.
(_After Macgillivray._)
(_aa_) Sclerotic; (_bb_) Choroid Coat and Pigment; (_c_) Ciliary
Circle; (_d_) Lens; (_ee_) Iris; (_f_) Cornea; (_g_) Optic Nerve;
(_i_) Pecten.
]
A third eyelid is well developed in this class; it is an elastic
membrane (_membrana nictitans_, or winking membrane), which has not,
like the other two, a vertical movement, but is drawn obliquely over
the eye from the inner to the outer side. This movement is effected
by two special muscles, one of which arises on the inside, and below
the eyeball, and has therefore to pass over to the outer side. In
contracting, it would press on the optic nerve, were it not for the
other one, which, however, is so disposed that by its contraction it
draws away the tendon of the _pyramidalis_ muscle from pressing on the
nerve. As in ourselves, there are six special muscles for moving the
orbit or ball of the eye, but the one which in man is well enough known
as the _trochlear_, has no pulley-arrangements in birds. Lachrymal
glands are present.
With regard to the organ of hearing, one particular part, which in man
is in the form of a snail’s shell, and is known as the _cochlea_, is
not coiled into this shape in birds, being very slightly bent, though
holding in other respects the same general relations. Nor is there any
external ear, as in mammals, for collecting the waves of sound; there
is, however, in the nocturnal birds of prey a crescent-shaped valve on
which are set tufts of short feathers, and it is possible that this
may aid in hearing. Nor, again, are there in the interior of the ear
those three small bones, which are known generally as the auditory
ossicles; of the two that are absent, one is thought by many anatomists
to be represented by the quadrate bone, which, as has already been
mentioned, connects the lower jaw of the bird with the skull. The
single bone which is present, and which is, perhaps, most generally
known as the “columella,” is connected by two or three cartilaginous
processes with the drum of the ear, and by the other end--at which it
has a small oval plate--with the more internal parts of the organ of
hearing. In man there is a curious arrangement of rods, which vary in
so remarkable a way as to have led to the supposition that each was
adapted to a distinct note; these rods, which constitute the organ of
Corti, are not present in birds, affording thereby a striking example
of the law that physiological inferences are often well examined by the
aid of comparative anatomy, no physiologist being hardy enough to deny
to birds the power of appreciating those delicate modulations of sound
which go to make up the chief charm of music. With regard to the organ
of smell, it is only necessary to note the absence of those muscles by
which, in man and other mammals, the nostrils are contracted or dilated.
The first point which attracts us on examining the digestive tract
of birds is the absence of lips and of teeth; but with regard to
these latter we must note that it is a character which has only
become distinct since the time when birds were first developed. This
statement is borne out by two series of facts, each taken from one of
the two great aids to a correct apprehension of the real importance of
structural characters--that is, from embryology, or the study of the
developing individual; and from palæontology, or the natural history of
the past. The young of certain Parrots have been observed to possess,
at an early stage of their development, those uprisings on the mucous
membrane of the jaw which go by the name of “dental papillæ,” and these
papillæ have been seen to be covered with a cap of dentine. On the
other hand, the researches of Owen and of some American palæontologists
have brought to light bird-like forms which were provided with teeth
(Odontornithes: _Ichthyornis_, _Hesperornis_).
The _beak_, or horny covering of the jaws, varies very greatly in
form, and in the degree of its sensibility. This tactile sense is
dependent on the extent to which the beak is supplied by nerves (from
the fifth cerebral nerve). In the Woodpecker, for example, there is a
large branch extending along the inside of the lower jaw, which, as it
approaches the extremity, breaks up into finer nerves that perforate
the bone by a number of small canals and so give to the beak a power
of discovering what lies hid in the crevices of the wood and under
the bark. Being an external structure, the beak is naturally adapted
to the habits of its possessors, so that it may be hooked, as in many
flesh-eating forms, or trenchant, and fit to cut and break, or provided
with transversely-set fine plates by which the water taken in with the
food can be filtered off, or provided with bristles, the better to hold
a living prey. Finally, in many cases the hardness of the bill is made
up for by a patch of naked skin at the base of the upper mandibles,
which is known by the name of the “cere” and seems to have a tactile
function.
In many birds, the tongue is either feebly developed, or is encased
in horn, so that it can hardly be as useful an organ of taste as is
our tongue: in the Pelicans it is obsolete. In some, however, as in
the Woodpecker, the tongue is a very powerful seizing organ, as it is
protruded with great rapidity by means of a special muscle, and is well
provided with a sticky secretion, which is given off from a large gland
(the sub-lingual), which, lying below the muscle above referred to,
is compressed when this muscle contracts; so that in the Woodpecker,
just as in the mammal called the Great Ant-eater (_Myrmecophaga_), the
insect prey is easily captured.
The region of the mouth is not separated from that which follows
it (the pharynx) by an epiglottis, which in ourselves protects the
entrance into the air-passages, nor is there any uvula to guard the
posterior orifice of the nose by which the air reaches the throat. The
succeeding portion of the gullet (the œsophagus) is very long, as might
be supposed from the length of the neck in most birds, and it is very
frequently either dilated at one side, or produced into a cæcal pouch
(crop, _ingluvies_), which may, or may not, be separated by a narrow
connection, from the rest of the gullet, and which may be divided into
two compartments. This crop serves for the detention of the food, which
cannot have undergone any complete process of mastication, and it is
here treated to a process of maceration by the fluid secreted from the
walls of this organ. Passing from this receptacle, the food becomes
subject to the action of the stomach proper, which differs, however,
from our ordinary conception of a stomach, as seen in man, by being
divided into two distinct portions. The anterior one is known as the
_proventriculus_, and it is in this that the gastric juice is brought
to bear upon the food, and its walls are consequently thickened by a
glandular layer; the hinder division, which is known as the _gizzard_,
forms an elongated sac, with two orifices--one from the proventriculus,
the other leading to the small intestine--in its upper portion. The
characters of its walls are very different in those birds which live on
animal, as compared with those that live on vegetable (grains) food;
in the former they are membranous and thin, but in the latter they are
enormously thick and very muscular. On examination, it is seen that the
dark colour of the muscles is on each side of the gizzard relieved by a
shining spot of tendinous material, and the walls of the gizzard have
consequently been compared to a double-bellied (digastric) muscle. The
internal cavity of the gizzard is lined with a dense and rough coat,
and is ordinarily found to contain small stones, and occasionally other
hard materials. These obviously take the place of the absent teeth,
when the muscles of the gizzard set up that (grinding or compressing)
action by which the ingested seeds are broken down. The wall of the
gizzard may itself also act as a rasping organ, being, as it often
is, provided with a firm glandular layer, the secretion of which is
converted into a hard lining, the structure of which has been observed
in some cases to be due to interlaced filaments secreted from and
continuous with the glands in the wall of the gizzard.
Notwithstanding the differences in the character of the gizzard
in carnivorous and graminivorous birds, it has been shown by the
ever-famous John Hunter that carnivorous birds can be brought to live
on grains, and grain-eating birds on meat.
It is interesting, further, to note, with regard to the opening into
the small intestine, that in a number of grain-or fruit-eating birds
there is no valvular arrangement to detain the food in the gizzard till
it is completely triturated, for it is thus that many plants have their
area of distribution increased, the escaped seeds passing uninjured
from the intestine to find, perhaps, a suitable soil in a new district.
In those that swallow large stones a valve is often to be observed.
The difference which we have already had so frequently to notice, as
obtaining between the carnivorous and “vegetarian” birds, is seen to be
continued into their small intestine; just as in mammals, this portion
of the tract is longer in the latter than in the former birds. The
anterior, or _duodenal_ portion, is always characterised by forming
a loop, within which lies the gland known as the _pancreas_, and the
succeeding portion is, as compared with most mammals, short. A slight
elevation, hardly ever of any great size, may at times be observed on
the course of the short intestine. This represents all that remains of
the duct by which the hatching bird was connected with the yolk. The
short and straight large intestine is ordinarily separated from the
preceding by a cæcum; this is generally paired (in the Herons and some
others it is single), and varies in length; in many cases these cæcal
tubes are hardly more than papillæ. In the Parrot, as in the Woodpecker
and some others, these cæca are absent. In the desert-dwelling Ostrich
(_Struthio_) they are said to be as much as two feet long; but in the
Emu they do not exceed six inches in length.
[Illustration:
DIGESTIVE ORGANS OF THE KINGFISHER.
(_After Macgillivray._)
(_a_) Tongue; (_b_, _c_, _d_) Œsophogus; (_d_, _e_, _f_) Stomach;
(_f_, _g_, _h_, _i_, _j_, _k_) Intestine; (_j_, _k_) Cloaca.
]
The intestine ends in a cavity, which is common to it, and to the other
organs that open to the exterior in this region. This _cloaca_ (sewer)
is found in reptiles also, and in one division of the Mammalia, the
Monotremata. In birds it is provided with a special glandular appendage
on its upper (or _dorsal_) aspect, which goes by the name of the _Bursa
Fabricii_. Neither the history nor the functions of this peculiar organ
can be said to be thoroughly understood.
Of the organs which are appended to the intestine, the lungs will be
described elsewhere; of the rest we have to consider the liver, the
pancreas, and the spleen. The first-named organ is large, and covers
over the pancreas, the proventriculus, the spleen, part of the gizzard,
and part of the small intestine. It is ordinarily divided into two
“lobes,” between which, on the upper edge, is placed the tip of the
heart. In the common fowl the left lobe is often divided into two; but
this organ is never broken up into so large a number of parts as it is
in many mammals, from which animals birds also differ in always having
more than one duct to carry off the secretion of the liver (bile) to
the small intestine, except in the Ostrich; in this, as in some other
birds, there is no gall-bladder in which the bile may be collected, so
that in such this secretion passes directly into the intestine.
As has been already pointed out, the commencement of the small
intestine forms a loop, in which is set the organ known as _pancreas_,
which may for simplicity be described as the salivary gland of this
region, although in truth the fluid secreted from it is a much more
powerful aid to the digestion of food than that of any known salivary
gland. It has always two, and in a number of cases three ducts, which
do not unite with the bile ducts, but open separately from, though near
them, into the end of the “duodenal loop.” The _spleen_, which is a
small oval body, and is placed to the right of the proventriculus, has
no ducts; in birds of prey it is more cylindrical in shape.
The temperature of the blood of the bird is, in requirement with the
conditions of its existence, hot--that is to say, it is ordinarily
hotter than the temperature of the surrounding air, and is found to
register between 100° (Gull) and 112° (Swallow) on Fahrenheit’s scale,
or from two to fourteen degrees more than does that of man. Birds
and mammals, are, speaking broadly, the only hot-blooded animals now
existing, and it has consequently been suggested that they should be
grouped together as such, in opposition to the rest of the Vertebrata.
But it is obvious that this character of the temperature is merely
dependent on physiological conditions; and were this a treatise on
the anatomy of birds rather than one on their natural history, the
statement of this fact would not receive the prominence here given to
it. The high temperature of any body may be preserved from cooling
influences by two methods: thus, tea in a well-polished silver teapot
keeps hot because the rays of heat are but slightly radiated from
its surface; or a less costly teapot may be kept hot by covering it
with a loosely-fitting “cosy,” which, being made of badly-conducting
materials, “keeps the heat in.” It is, then, clear that the heat of a
body is best preserved when it is covered by a bad radiator and a bad
conductor of heat; and this is just the case with birds: the polished
feathers are bad radiators, and the air entangled among them forms a
bad conductor.
The blood corpuscles are, broadly speaking, about twice as large as in
man; those which are coloured red are oval in shape, as they are in
nearly all of the lower Vertebrates and in the Camels among mammals.
Like the white ones, they are “nucleated.” The heart is, as in mammals,
divided into four chambers. It is a condition of the circulation
in hot-blooded and rapidly-breathing animals that the current of
_arterial_ blood _from_ the heart, and the current of _venous_ blood
_to_ it, should be kept as much as possible separate; no reflection is
needed to show that the blood freshly purified by contact with the air
in the lungs must be kept as distinct as can be from the blood which
has lost its purity in passing through the body; in other words, it is
required that there should be a similar result in birds and in mammals.
Birds, like all warm-blooded creatures, have the heart divided into
four cavities--two ventricles and two auricles--those of the right
side being completely separated from those of the left. The whole is
enclosed in a pericardium, a thin, but strong, membrane. The right
ventricle has thin muscular walls, and almost completely envelopes
the left. The right auricle has a remarkable valve in the shape of a
fleshy leaflet, which appears almost to be a portion of the inside
of the ventricle that has become detached from the partition between
the two ventricles. The blood, under certain circumstances, passes
between this septum, or partition, and the leaflet, into the auricle;
but when the beat of the heart takes place (the systole), the septum,
being convex, is forced against the leaflet on the other side of the
auricolo-ventricular opening, and the passage of the blood, through
this, is prevented. The valve between the stout-chambered left
ventricle and auricle does not present this structure, but is divided
into two or three lobes attached to tendinous processes. At the origin
of the great vessels--the pulmonary artery and the aorta--there are
three valves, semi-lunar in shape and by name. And this last vessel,
often having given off the coronary artery to the heart itself, is
curved to the right, and then passes backwards to go down the body. The
blood from the body is collected into three large veins--two anterior
_venæ cavæ_ and one posterior.
The lymphatic system is well developed, and of the so-called “lymphatic
hearts,” which are well known in the Frog, the posterior ones have been
observed in some, and especially in the Ratite birds.
The lungs, or organs in which the blood effects an exchange of its
gases with the outer air, are paired, and set on either side of the
heart. As is elsewhere mentioned, the nostrils are not provided with
muscles, and there is no _epiglottis_ sufficiently well developed to
cover the entrance into the long tube, or _trachea_, which runs down
the neck. This tube, which does not always take a straight course,
is essentially made up of a number of rings of cartilage, which are
for the greater part perfect, and not, as in man, imperfect rings.
The _bronchi_ which are given off from this tube, to the right and
left, have their rings imperfect, and they do not show that two-forked
mode of division which is so characteristic of mammals. The lungs
are of a rosy colour, and of a comparatively small volume; they are
marked externally by depressions corresponding to the characters of
the vertebræ and ribs, to which latter they are firmly attached, and
they are not divided into _lobes_; in their texture they are spongy;
the air-tubes are given off from them at right angles to the main
air-passage; these run nearly parallel to one another, and contain in
their walls the true tissue of the respiratory organ. The air-tubes are
also connected with the air-cells, which are arranged in so remarkable
a manner as to deserve a full account.
They are found in all birds with the exception of the _Apteryx_,
according to Professor Owen. Our knowledge of their existence is
primarily due to the illustrious William Harvey, while it is to the
distinguished anatomist, John Hunter, that we owe our knowledge of the
very curious fact that these air-passages and sacs communicate also
with the cavities of some of the bones of the skeleton. Though these
sacs are not by any means highly vascular, or supplied with vessels
to the same rich extent as are the lungs, they are nevertheless of
enormous importance to the bird; thus, they diminish the specific
gravity of the animal. For example, taking a bird which weighs 1,600
grammes, and has a volume of 1,230 cubic centimetres--or a specific
gravity of 1·30 (1600/1230) it has been calculated (Bert) that 200
cubic centimetres of air can be introduced; now these centimetres would
weigh ·22 of a gramme, so that the specific gravity of the animal
would be reduced to 1·05 ((1600+0·22)/(1230+200)) or (1600·22/1520).
Again, the air which is taken into the lungs is, in high-flying
birds, often of an extremely low temperature; but this air is not
only brought into contact with that of the lungs, but also with that
which has been warmed in the abdominal cavity. And again, the air
is often very dry--as it is for the Ostrich on the desert plains of
Africa--but the air from the air-sacs contains a large amount of
moisture. Of the proper air-sacs there are nine; of these, four--the
two anterior and the two posterior _thoracic_--lie in the thorax
(breast) proper; three--the right and left cervical, and the sac
between the _clavicles_--lie in front of the thorax; while the last
two are found behind it and in the abdomen. From all of these, with
the exception of those within the thorax, communications are, or may
be, given off to the bones of the vertebral column, to the humerus, to
the bones of the thigh, and to the sternum and the ribs; but there is
no communication between these sacs and the air-spaces which are so
constantly found in the bones of the skull, and which are in connection
with the air-cavities of the ear and of the nose. The inter-clavicular
sac has been observed to be covered with a thick layer of muscle in
those birds, at any rate, which perform somersaults, and it has been
suggested that this layer of muscle is capable of driving the air in
the sac backwards. It is obvious that such an operation would send the
centre of gravity of the animal nearer the head, and would, so far, be
of assistance in the execution of the curious movement alluded to.
It has been suggested that the air-sacs are of assistance in increasing
the resonance of the bird’s voice. Be this as it may, attention must
now be turned to the organ of voice. This organ may take one of three
forms, or, if absence is to be counted, four. There is no organ of
voice in the Ratitæ, or in the American Vultures (_Cathartidæ_). It is,
when present, remarkable for being developed at the lower, and not at
the upper, end of the _trachea_; while the true _vocal cords_, which,
by their vibration produce the notes of the human voice, are altogether
and always absent from the larynx; in other words, the vocal organ is
not the _larynx_, but an organ seated at a lower level, and known as
the _syrinx_. This instrument may, further, be formed in the trachea
alone (as in some American Passerines), or in the bronchi alone (as in
Steatornis), or at the point at which the tracheal and bronchial tubes
pass into one another (as in the majority of singing birds).
The last-mentioned, or _bronchio-tracheal syrinx_, consists of the
following parts; (i.) a _tympanic_ chamber formed by the union of
some of the lower rings of the trachea; (ii.) a membranous _septum_
separating from one another the tracheal orifices of the two bronchi;
(iii.) on either side a _tympaniform membrane_, formed on the inner
side of the uppermost bronchial rings; in consequence of this these
bronchial rings are not complete circles; their mucous membrane is
developed into a fold which bounds one side of a cleft which is formed
by the presence on the other side of the above-mentioned tympaniform
membrane. The air which passes through these bronchial clefts sets in
vibration the membranes which bound them, while the character of the
note is affected by the position of the bronchial half-rings, and the
length of the column of air in the trachea. These rings have their
positions changed by five lateral muscles, which act on their ends,
and so rotate them. The principle variations in the characters of the
muscular supply of the organ of the voice were long ago worked out by
Johannes Müller, the famous German anatomist and physiologist.
It is also to this observer that we owe our first information with
regard to the bronchial syrinx of Steatornis; the anatomy of this
animal was also investigated by the late Prof. Garrod, who gave the
following account of its vocal apparatus:--“Each semi-syrinx, as it
may be termed, is formed on the same principle as that of the combined
organ in most of the non-singing birds. Taking for description that
of the left side, it is found that the thirteenth bronchial ring
is complete, though considerably flattened from side to side; the
fourteenth is not complete in the middle of its upper surface; it
is a little longer from before backwards than the one above, and
not so long as the one following it. The fifteenth is only a half
ring, its inner portion being deficient; it is slightly convex
upwards, and articulates, both at its anterior and posterior ends,
with the fourteenth incomplete ring and the sixteenth half-ring. The
sixteenth half-ring is concave upwards, and so forms an oval figure in
combination with the one above, which is filled with a thin membrane
to form part of the outer wall of the bronchus. There is a membrane
also between the ends of these and the succeeding half-rings, which
completes the tube of the bronchus internally.”
[Illustration:
FRONT VIEW (A) AND SECTION (B) OF INFERIOR LARYNX OF PEREGRINE FALCON.
(_After Macgillivray._)]
The ducts from the urinary organs open to the exterior through the
cloaca, into which, as already mentioned, the digestive tube also
opens. The chief point with regard to the urinary secretion of birds
is the fact that it is _semi-solid_, and that it contains a quantity
of the substance known as _uric acid_. The kidneys are placed some way
back and near the cloaca; they are set on either side of the spinal
column, between the transverse processes of the sacral vertebræ, and
are generally divided into three portions of greatly varying size. On
their inner edge are given off the _ureters_, which pass on each side
to enter separately into the before-mentioned cloaca.
The right ovary of birds is always atrophied, and it is in rare cases
only that rudiments of it are found (namely, in the diurnal Raptores).
The _oviduct_ is a coiled canal, the lower portion of which has strong,
muscular walls, while internally the characters of its surface vary
according to the substance which the glands of different regions add to
the descending egg. The right oviduct is not so completely atrophied as
is the ovary of the same side. This duct opens into the cloaca through
which the egg passes to reach the outer world; as further development
is so largely independent of the mother, the female organs offer no
peculiarities of arrangement, or complexities of structure.
All birds lay eggs, or, in other words, the born young are not carried
about by the mother till the time of birth. The advantage of this to a
flying animal is so obvious that we may pass at once to describe the
egg of a common fowl. The shell, which consists of organic matter and
lime-salts, is found to be formed of two layers; it is in the outer
one only that pigment is found. Both layers are traversed by canals,
through which air can pass only when the shell is dry; that is to
say, the outer pores of the shell are closed under the influence of
moisture. This may be seen by removing the outer layers, when air or
water will pass in quite easily. These canals are said to be branched
in the Ratite birds, and to be simple in the Carinatæ. The shell is
lined by the shell-membrane, which, again, is made up of two layers.
At the broad end of the egg these two layers are separated from one
another, and so give rise to that air-chamber which is found in stale
eggs, and increases in size as the egg grows older and the yolk
evaporates.
The shell-membrane is in direct contact with the _white_ of the egg
(_albumen_). This, in its fresh state, consists of fluid albumen,
arranged in layers, which are separated from one another by networks
of fibres, in the meshes of which, however, fluid albumen is also to
be found. There are, further, two special sets of fibrous cords in the
white of the egg; these extend somewhat along the long axis of the egg,
though they do not reach to the shell-membrane. From their bead-like
character they are known as _chalazæ_ (hailstones), but their more
common English name is that of the “tread.”
[Illustration: DIAGRAMMATIC SECTION OF A FOWL’S EGG.
(_bl_) Blastoderm; (_wy_) White Yolk; (_yy_) Yellow Yolk; (_vt_)
Vitelline Membrane; (_w_) Albumen; (_ch_) Chalazæ; (_ach_)
Air-chamber; (_ism_) Internal Layer of Shell Membrane; (_em_)
External Layer of ditto; (_s_) Shell.
]
The “white” is separated from the _yolk_ by the so-called _vitelline_
(or _yolk_) membrane; the greater part of this yolk is known as the
_yellow yolk_, and is made up of minute albuminous granules, but its
outermost part is formed of a thin layer of a somewhat different
substance, which goes by the name of the _white yolk_. The spheres of
this latter are still smaller than those of the yellow yolk, and they
are also found to form layers at various levels in it. At one point
the white yolk becomes a good deal thicker, and forms, as it were,
a pad for a small white disc, which, in ordinary circumstances, is
always found uppermost when an egg is opened. This disc is formed of
an encircling white rim, and within it there is a rounded transparent
region, the centre of which is more opaque.
This region is known as the _blastoderm_, and is that part of the egg
from which the chick, with its organs and complicated vessels, muscles
and bones is soon to be developed. In the laid egg, this blastoderm
consists of two layers of cells, as do at a certain stage the eggs of
all but the very simplest of animals. The dissection of a laying fowl
will probably reveal the presence of eggs at an earlier stage, and from
their study the following history has been made out: the ellipse-shaped
egg, when about to leave the ovary, is a yellow body enclosed in a
fine membrane, and possessing at one pole a small (_germinal_) disc;
this disc contains a smaller _germinal vesicle_, and a still smaller
_germinal spot_; when this body is ripe, it escapes from its enclosing
capsule, and the germinal vesicle disappears. As the egg passes down
the _oviduct_ the albumen becomes deposited around it, and part of it
is converted into the shell-membrane. The egg now becomes subjected to
a thick, white fluid, which is gradually converted into the shell.
While these additions to the substance of the egg are going on, the
germinal disc undergoes the remarkable process known as segmentation,
in which it becomes divided into two, four, eight, sixteen, thirty-two
(and so on) masses, which arrange themselves in two distinct layers,
the presence of which has been already noted in the laid egg.
This is not the place in which it is possible to follow out the various
future changes undergone, but the condition of the young birds on
escaping from the egg is widely different in some of the larger groups
of birds. Some young birds, on their exclusion from the egg, are able
to shift for themselves, and are covered with down; while others are
born naked and helpless, and require food from their parents for some
time after they are hatched. Of the first section, an ordinary chicken
is a familiar example, while a young Thrush or a Sparrow illustrates
the second. There are, however, manifest exceptions to this rule, as
in the Herons, for instance, where the young are densely clothed with
feathery down, but are helpless for a long time after they are hatched.
Finally, it must be stated that all birds possess an oil-gland (known
as the uropygial), situated near the tail, with which they clean and
dress their feathers. Attention has already been called to this gland
in the foot note on p. 245.
Before commencing the special part of the present article, it may
be useful to give a slight sketch of the classification which it is
proposed to follow throughout its course.
CLASS AVES.
DIVISION I. CARINATÆ: CARINATE BIRDS.
ORDER I.--ACCIPITRES: BIRDS OF PREY.
SUB-ORDER I.--FALCONES: FALCONS.
Family I.--Vulturidæ Vultures.
„ II.--Falconidæ Hawks.
Sub-Family I.--Polyborinæ Caracaras.
„ II.--Accipitrinæ Long-legged Hawks.
„ III.--Buteoninæ Buzzards.
„ IV.--Aquilinæ Eagles.
„ V.--Falconinæ Falcons.
SUB-ORDER II.--PANDIONES: OSPREYS.
SUB-ORDER III.--STRIGES: OWLS.
Family I.--Bubonidæ Owls proper.
„ II.--Strigidæ Barn Owls.
ORDER II.--PICARIÆ: PICARIAN BIRDS.
SUB-ORDER I.--ZYGODACTYLÆ: CLIMBING BIRDS.
Family I.--Psittaci Parrots.
a.--_Psittaci proprii._
Sub-Family I.--Camptolophinæ Cockatoos.
„ II.--Androglossinæ Fleshy-tongued Parrots.
„ III.--Conurinæ Conures.
„ IV.--Platycercinæ Parrakeets.
„ V.--Strigopinæ Owl Parrots.
b.--_Psittaci orthognathi._
„ VI.--Trichoglossinæ Brush-tongued Parrots.
Family II.--Cuculidæ Cuckoos.
„ III.--Indicatoridæ Honey-guides.
„ IV.--Musophagidæ Touracoes.
„ V.--Picidæ Woodpeckers.
„ VI.--Rhamphastidæ Toucans.
„ VII.--Capitonidæ Barbets.
SUB-ORDER II.--FISSIROSTRES: WIDE GAPING BIRDS.
Family I.--Galbulidæ Jacamars.
„ II.--Bucconidæ Puff Birds.
„ III.--Alcedinidæ Kingfishers.
„ IV.--Bucerotidæ Hornbills.
„ V.--Upupidæ Hoopoes.
„ VI.--Meropidæ Bee-eaters.
„ VII.--Momotidæ Motmots.
„ VIII.--Coraciadæ Rollers.
„ IX.--Trogonidæ Trogons.
„ X.--Caprimulgidæ Goatsuckers.
„ XI.--Cypselidæ Swifts.
„ XII.--Trochilidæ Humming-birds.
ORDER III.--PASSERIFORMES: PERCHING BIRDS.
SECTION A.--ACROMYODI: SINGING BIRDS.
SUB-ORDER I.--TURDIFORMES: THRUSH LIKE BIRDS.
GROUP I.--COLIOMORPHÆ: CROW LIKE PASSERES.
Family I.--Corvidæ Crows.
Sub-Family I.--Corvinæ Crows proper.
„ II.--Fregilinæ Choughs.
Family II.--Paradisiidæ Birds of Paradise.
„ III.--Orioliidæ Orioles.
„ IV.--Dicruridæ Drongos.
„ V.--Prionopidæ Wood-shrikes.
GROUP II.--CICHLOMORPHÆ: THRUSH-LIKE PASSERES.
Family VI.--Campophagidæ - - - - Cuckoo-shrikes.
„ VII.--Muscicapidæ - - - - - Flycatchers.
„ VIII.--Turdidæ - - - - - - - True Thrushes.
Sub-Family I.--Turdinæ - - - - Thrushes.
„ II.--Sylviinæ - - - - Warblers.
Family IX.--Timeliidæ - - - - - - Babbling Thrushes.
Sub-Family I.--Troglodytinæ - - Wrens.
„ II.--Brachypodinæ - - Bulbuls.
„ III.--Timeliinæ - - - Babblers.
„ IV.--Cisticolinæ - - Grass-warblers.
„ V.--Miminæ - - - - - American Babblers.
Family X.--Laniidæ - - - - - - - Butcher-birds.
„ XI.--Vireonidæ - - - - - - Greenlets.
„ XII.--Paridæ - - - - - - - Titmice.
GROUP III.--CERTHIIMORPHÆ: CREEPERS.
GROUP IV.--CINNYRIMORPHÆ: HONEY SUCKERS.
SUB-ORDER II.--FRINGILLIFORMES: FINCH-LIKE BIRDS.
Family I.--Motacillidæ - - - - - Wagtails.
„ II.--Mniotiltidæ - - - - - American Warblers.
„ III.--Cærebidæ - - - - - American Creepers.
„ IV.--Diceidæ - - - - - - - Flower-peckers.
„ V.--Ampelidæ - - - - - - Chatterers.
„ VI.--Hirundinidæ - - - - - Swallows.
„ VII.--Tanagridæ - - - - - - Tanagers.
„ VIII.--Fringillidæ - - - - - Finches.
„ IX.--Icteridæ - - - - - Hang-nests.
SUB-ORDER III.--STURNIFORMES: STARLING-LIKE BIRDS.
Family I.--Ploceidæ - - - - - - Weavers.
„ II.--Sturnidæ - - - - - - Starlings.
„ III.--Artamidæ - - - - - - Wood-swallows.
„ IV.--Alaudidæ - - - - - - Larks.
SECTION B.--MESOMYODI: SONGLESS BIRDS.
Family I.--Menuridæ - - - - - - Lyre-birds.
„ II.--Pteroptochidæ - - - - Bush-wrens.
„ III.--Dendrocolaptidæ - - - Spine-tails.
„ IV.--Formicariidæ - - - { American Ant-thrushes.
„ V.--Pittidæ - - - - - - { Old-World Ant-thrushes.
„ VI.--Tyrannidæ - - - - - - Tyrant-birds.
„ VII.--Cotingidæ - - - - - - American Chatterers.
„ VIII.--Pipridæ - - - - - - - Manakins.
„ IX.--Eurylæmiidæ - - - - - Broadbills.
„ X.--Phytotomidæ - - - - - Plant-cutters.
ORDER IV.--COLUMBÆ: PIGEONS.
ORDER V.--GALLINÆ: GAME-BIRDS.
Family I.--Cracidæ - - - - - - - Curassows.
„ II.--Opisthocomidæ - - - - Hoatzins.
„ III.--Phasianidæ - - - - - Pheasants.
„ IV.--Meleagridæ - - - - - Turkeys.
„ V.--Tetraonidæ - - - - - Grouse.
„ VI.--Pteroclidæ - - - - - Sand-grouse.
„ VII.--Turnicidæ - - - - - - Hemipodes.
„ VIII.--Megapodidæ - - - - - Megapodes.
ORDER VI.--GRALLÆ: WADING BIRDS.
Family I.--Rallidæ - - - - - - - Rails.
„ II.--Scolopacidæ - - - - - Snipes.
„ III.--Charadriidæ - - - - - Plovers.
„ IV.--Otididæ - - - - - - - Bustards.
„ V.--Gruidæ - - - - - - - Cranes.
„ VI.--Psophiidæ - - - - - - Trumpeters.
ORDER VII.--HERODIONES: HERONS.
Family I.--Ardeidæ - - - - - - - Herons proper.
„ II.--Ciconiidæ - - - - - - Storks.
„ III.--Plataleidæ - - - - - Spoonbills.
„ IV.--Phænicopteridæ - - - Flamingoes.
ORDER VIII.--ANSERES: GEESE.
Family I.--Palamedeidæ - - - - - Screamers.
„ II.--Anatidæ - - - - - - - Ducks.
ORDER IX.--STEGANOPODES: PELICANS.
Family I.--Fregatidæ - - - - - - Frigate-birds.
„ II.--Phæthontidæ - - - - - Tropic-birds.
„ III.--Pelecanidæ - - - - - Pelicans.
ORDER X.--GAVIÆ: SEA-BIRDS.
Family I.--Laridæ - - - - - - - Gulls.
„ II.--Procellariidæ - - - - Petrels.
ORDER XI.--PYGOPODES: GREBES.
ORDER XII.--IMPENNES: PENGUINS.
ORDER XIII.--CRYPTURI: TINAMOUS.
DIVISION II.--RATITÆ: STRUTHIOUS BIRDS.
DIVISION III.--SAURURÆ: LIZARD-TAILED BIRDS.
It has been already stated that birds are divisible into three great
sections, and attention is now directed to those which have a keel to
the sternum, and which are good flyers--the Carinate Birds (CARINATÆ).