Air Sacs

Pulmonary and Nasopharyngeal Systems in Birds

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.


Home to the Wonder of Birds

This page ©