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Air sacs are membranaceous receptacles
which communicate with the cavities of the respiratory organs or passages,
and can through them be filled with air. According to their connections
we distinguish between a (I) pulmonary and (II) a nasopharyngeal system
of air-sacs.
I. The pulmonary system has the widest
distribution in the bird's body. The sacs, of which there are generally
five large pairs, begin in the embryo of about eleven days to grow out
as small vesicles from the surface of the lungs, as dilatations of branches
of the bronchial tubes, pushing the peritoneal membranes before them, and
gradually extending as enlarged sacs into the body cavity between the various
intestines. Each sac has an inner layer, the continuation of the lining
membrane of the bronchial tubes, and an outer layer or serous membrane,
which is the bulged-out pleura or peritoneal covering of the lung. The
pulmonary openings are beset with vibrating ciliae like the bronchi. The
outside of the sacs frequently possesses a covering of involuntary or of
voluntary muscles; for instance, in Vultures, Gannets, and Flamingos a
thin fan-shaped muscle extends from the furcula over the interclavicular
air-sac. Through contraction of these muscles the cells can be emptied
of air. The five principal pairs of air-sacs are:-
1. A prebronchial or cervical
pair, situated in front of or "headwards" from the lungs and the pulmonary
system. They are subjected to many modifications. They form on each side
a single sac in the Duck, which in the Fowls, Gulls, Gannets, and some
others, communicates with the next pair. In the Stork, Flamingo, and Screamer
each sac is elongated and divided into numerous smaller cells. Frequently
these sacs extend far up the neck, even into the head, and small side branches
may enter any of the neighbouring organs, such as the inside
of the vertebrae, the carotid and vertebral canals, the cervical
muscles, the cranial cavities, and others. Sometimes they form large inflatable
sacs on the throat, as, for instance, in the Prairie-fowls.
2. A pair of subbronchial
or interclavicular sacs. They are united into one sac in Storks, communicate
with each other in Ducks, are subdivided into a number of smaller
sacs in the Swan and in the Screamer: in Vultures they take the large crop
between them. Lateral extensions accompany the large blood-vessels and
form axillary cells penetrating ultimately the humerus and other bones
of the wing; other secondary cells penetrate the large pectoral
muscles (e.g. in Mycteria) or enter the body and the keel of
the sternum.
3 and 4. A pair of anterior
and posterior intermediate sacs, extending more or less far into the abdominal
cavity, covering chiefly the lower portions of the lungs and the
liver, occasionally subdivided, being filled through several openings,
at the external edge of the lungs, and sometimes continued into
the lateral parts of the sternum.
5. A pair of abdominal sacs. These
are the largest, extending with irregular subdivisions between the intestines
into the pelvis, and penetrating the femur together with the rest of
the bones ofthe sacrum, and the legs.
Besides these principal air-sacs, there exist
numerous smaller cells, which enter more or less directly from the lungs
into the vertebrae and ribs, between the muscles, underneath the skin and
other parts, thus making the skeleton, and sometimes the greater part of
the body, pneumatic. The air-sacs do not enter the bones before a considerable
portion ofthe marrow has been absorbed, an extremely small hole in the
bone is sufficient for their entrance; the cavity of hollow bones
is ultimately lined with the thin membrane of the air-sac. Generally
the skeleton is most pneumatic in large birds that fly well, like Vultures,
Storks, Swans, Pelicans; less so in small birds, and least in heavy or
little-flying water-birds. However, there are many exceptions. While, for
instance, most of the bones of many Passeres, of Swifts,
Divers, Rails, the Kiwi, and of Terns, are solid, and air-cells
are restricted chiefly to the cranium, many parts of the skeleton
of the large Ratitae are very pneumatic.
The greatest development of pneumatic
cells exists in the Screamers and Hornbills, in which even the fingers
and toes, in fact, any part ofthe skeleton, are hollow, and large subcutaneous
air-sacs are present in great numbers between the muscles and the roots
of the feathers. These birds when inflated and pricked emit a peculiar
hissing noise through the skin. It is well known that a bird which has
its humerus shattered by shot can for some time breathe, although
its beak and nostrils be tightly closed, and thus be submitted to unnecessary
excruciating pain. Compression of the thorax and abdomen suffocates a wounded
bird more efficiently than strangulation.
II. The naso-pharyngeal or tympanic system
of air-sacs is restricted to the head, extending chiefly into the
occipital, frontal, parietal, quadrate, and mandibular bones, To this system
belong the Eustachian tubes (see Ear and Skull), the tympanic, and other
cavities which communicate with the nose. The most curious dilatation belonging
to this system is the crop-like pouch of the Adjutant.
This sac communicates in Leptoptilus crumenifer
with a large cavity
below the orbit and the pterygoid bone on the left side of the basis
cranii, opening directly into the nasal cavity and extending like a hernia
into a loose fold of integument, the pouch being divided into two
by a vertical membrane which descends to the level of the eighth
cervical vertebra.
Another inflatable sac is the gular pouch
of Bustards. It seems to be developed only in adult males, reaching
its greatest size during the breeding season, and again shrivelling up
during the rest of the year. Its opening is a  shaped
slit in front of the frenulum of the tongue and below this organ;
the opening can be closed by muscles, and leads into a large, glandless
blind sac (about 8-10 inches long, with half the width), which is a dilatation
of the frenulum and hangs down between the throat and the skin ofthe
front of the neck. It seems to be an entirely sexual ornament, inflating
the skin, and containing neither water nor food.
A similar homologous structure exists in
the male of Biziura lobata,
as a little pouch between the two halves
of the frenulum, with a roundish opening, but apparently not extending
into or inflating the outer cutaneous wattle or fold underneath the mandibles.
Lastly, the tracheal pouch of the Emu may
be mentioned. It is a large unpaired hernia-like sac of the tracheal
walls, communicating with the trachea through a longitudinal slit on the
ventral side, an individually-varying number of from five to fourteen
cartilaginous rings being known to be deficient in the middle line. In
the embryos this deficiency is already shown, but the pouch is developed
much later, and attains its full size in the adults of both sexes.
This organ seems to act as a resounding bag to the peculiar drumming noise
made by the adult birds.
The function of all these air-sacs
has been the subject of many controversies. Some are undoubtedly subservient
to sexual omamental purposes, by inflating the skin, rustling the feathers,
or acting as resounding bags in the Prairie-fowls and in the Emu. The suggestion
that the warm air in these sacs makes the bird lighter, and assists, balloon-like,
the flight, is void of practical value, because the few grains of weight
lifted up by the whole amount of air-sacs of even a large bird would be
more than counter-balanced by a few grains of food or better-nourished
condition of the bird. Nor would this view be applicable to the Ratitre,
with their well-developed air-sacs. One of the principal functions of the
air-sacs consists in the ventilation of the lungs, the latter being only
capable of very limited expansion and contraction in birds. No exchange
of gas seems to take place in the sacs themselves, they being poor in blood-vessels;
but they seem to be directly connected with the regulation of the exhalation
of aqueous vapour, there being besides no perspiration through the skin.
Frequently they serve also as reservoirs for air, in order to increase
the voice; for instance, in the long-continued song of the Nightingale,
or still more so, in the Lark when warbling.
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