The _lungs_ are small and occupy only the dorsal portion of the thoracic
cavity. There is only one right and one left lobe, each traversed
through its whole length by a _mesobronchium_, whence arise about ten
secondary bronchia; these send off radially arranged _parabronchia_,
which end blindly near the surface. The walls of these tertiary tubes
send out, in all directions, _canaliculi aeriferi_ which, ending in
slight swellings, recall the mammalian _aveoli_.
Highly specialized air-sacs are characteristic of all birds. They are
very thin-walled membranes, very poor in blood-vessels, formed by the
bulged-out pleural or peritoneal covering of the lungs, through the
parabronchial tubes of which they are filled with air. Their function is
not quite clear. The usual suggestion, that the warm air contained
within them assists the bird in flight, balloon-like, is absurd. They
assist in the extremely rapid and vigorous ventilation of the lungs, the
latter being capable of but very limited expansion and contraction in
birds. Exchange of gas through the walls of the air-sacs, almost devoid
of blood-vessels, can at best be much restricted.
There are five pairs of larger sacs belonging to the pulmonary
system:--(1) prebronchial or cervical, extending sometimes far up the
neck, even into the cranial cavities; the throat-bags of the prairie
fowls (_Cupidonia_ and _Pedioecetes_) are a further development; (2)
subbronchial or interclavicular; (3 and 4) anterior and posterior
thoracic or intermediate; (5) abdominal sacs. Most of these extend
through narrow apertures--_foramina pneumatica_--into the hollow
bones, sometimes, e.g. in hornbills and screamers, into every part of
the skeleton, or, in the shape of innumerable pneumatic cells, even
beneath the skin. There is also a naso-pharyngeal or tympanic system
of air-sacs, restricted to the head (cf. the _siphonium_ described in
connexion with the mandible), but filling also such curious organs as
the frontal excrescence of _Chasmorhynchus_, the Brazilian bell-bird,
the throat-bag of the adjutant stork, and the gular pouch of the
bustard.
The _trachea_ or windpipe is strengthened by numerous cartilaginous,
often osseous, complete rings, but in the emeu several of these rings
are incomplete in the medioventral line, and permit the inner lining
of the trachea to bulge out into a large neck-pouch, which is used by
both sexes as a resounding bag. In humming-birds and petrels the
trachea is partly divided by a vertical, longitudinal, cartilaginous
septum. In some of those birds which have a peculiarly harsh or
trumpeting voice, the trachea is lengthened, forming loops which lie
subcutaneously (capercally, curassow), or it enters and dilates the
symphysis of the furcula (crested guineafowl); or, e.g. in the cranes
and in the hooper swan, even the whole crest of the sternum becomes
invaded by the much elongated, manifolded trachea.
The _syrinx_ or lower larynx is the most interesting and absolutely
avine modification, although absent as a voice-producing organ
(probably due to retrogression) in most Ratitae, storks, turkey
buzzards (_Cathartes_) and Steganopodes. The syrinx is a modification
of the lower part of the trachea and of the adjoining bronchi.
Essential are vibrating membranes between the cartilaginous framework,
and next, special muscles for regulating the tension. The majority of
birds possess a pair of internal tympaniform membranes forming the
inner or median walls of the bronchi, which are there furnished with
semi-rings only. External tympaniform membranes exist, with great
variations, between the specialized one or two last tracheal and some
of the first bronchial rings.
According to the position of the chief sound-producing membranes,
three types of syrinx are distinguishable:--(1) Tracheo-bronchial, by
far the commonest form, of which the two others are to a certain
extent modifications. The essential feature is that the proximal end
of the inner membranes is attached to the last pair of tracheal rings;
outer tympaniform membranes exist generally between the 2nd, 3rd and
4th bronchial semi-rings. This type attains its highest development in
the Oscines, but it occurs also in many other orders. (2) Syrinx
_bronchialis_. The outer membranes are spread out between two or more
successive bronchial semi-rings, a distance from the trachea which is,
in typical cases, devoid of sounding membranes; some Cuculi,
Caprimulgi, and some owls. (3) Syrinx _trachealis_. The lower portion
of the trachea consists of thin membranes, about half a dozen of the
rings being very thin or deficient. Inner and outer membranes may
exist on the bronchi. The _Tracheophonae_ among the Passeriformes, the
possessors of this specialized although low type of syrinx, form a
tolerably well-marked group, entirely neotropical. But indications of
such a syrinx occur also in _Pittidae_, pigeons and gallinaceous birds
(_Gallidae_), the last cases being clearly analogous.
Whilst the type of syrinx affords no help in classification, it is
very different with its muscles. These--as indicated by their supply
from a branch of the hypoglossal nerve, which descends on either side
of the trachea--are, so to speak, a detached, now mostly independent
colony of glosso-pharyngeal muscles. Omitting the paired
tracheo-clavicular muscles, we restrict ourselves to the syringeal
proper, those which extend between tracheal and bronchial rings. Their
numbers vary from one pair to seven, and they are inserted either upon
the middle portion of the bronchial semi-rings (_Mesomyodi_), or upon
the ends of these semi-rings where these pass into the inner
tympaniform membrane (_Acromyodi_). The former is morphologically the
more primitive condition, and is found in the overwhelming majority of
birds, including many Passeriformes. The acromyodian type is
restricted almost entirely to the Oscines. Further, according to these
muscles being inserted only upon the dorsal, or only upon the ventral,
or on both ends of the semi-rings, we distinguish between _an_-,
_kat_- and _diacromyodi_. But the distinction between such _Acromyodi_
and the _Mesomyodi_ is not always safe. For instance, the _Tyranninae_
are anacromyod, while the closely allied Pipras and Cotingas are
katacromyod; both these modifications can be shown to have been
derived but recently from the weak meso- and oligomyodian condition
which prevails in the majority of the so-called _Oligomyodi_. On the
other hand, the diacromyodian type can have been developed only from a
strong muscular basis which could split into a dorsal and a ventral
mass; moreover, no Passeres are known to be intermediate between those
that are diacromyodian and those that are not.
Attempts to derive the anacromyodian and the katacromyodian from the
diacromyodian condition are easy on paper, but quite hopeless when
hampered by the knowledge of anatomical facts and how to use them.
There remains but one logical way, namely, to distinguish as
follows:--(1) _Passeres anisomyodi_, in which the syrinx muscles are
unequally inserted, either on the middle or on one end of the
semi-rings, either dorsal or ventral. This type comprises the
Clamatores. (2) _Passeres diacromyodi_, in which some of the syrinx
muscles are attached to the dorsal, and some to the ventral ends,
those ends being, so to say, equally treated. This type comprises the
Oscines. Both types represent rather two divergent lines than
successive stages, although that of the Clamatores remains at a lower
level, possessing at the utmost three pairs of muscles, whilst these
range in the Oscines from rarely two or three to five or seven.
This way of using the characters of the syrinx for the classification
of the Passeriformes seems simple, but it took a long time to
accomplish. Joh. Mueller introduced the terms _Polymyodi_ and
_Tracheaphones_, Huxley that of _Oligomyodi_; Mueller himself had,
moreover, pointed out the more important characters of the mode of
insertion, but it was Garrod who invented the corresponding terms of
_Acro_-and _Mesomyodi_ (= _Tracheophones_ + _Oligomyodi_). (For
further historical detail, see ORNITHOLOGY). After W.A. Forbes had
investigated such important genera as _Philepitta_ and _Xenicus_, P.L.
Sclater, A. Newton and R.B. Sharpe divided the Passeres respectively
into _Oscines, Oligomyodae, Tracheophonae_ and _Pseudoscines_
(= _Suboscines_); _Oligomyodae, Tracheophonae_ and _Acromyodae;
Oscines, Oligomyodae, Tracheophonae_ and _Atrichiidae_. Ignoring the
fact that some _Oligomyodae_ are meso- and others acromyodian, they
tried to combine two irreconcilable principles, namely, mere numbers
against quality.
BIBLIOGRAPHY.--M. Baer, "Beitr. z. Kenntniss d. Atemwerkzeuge bei den
Voegeln," _Zeitschr. wiss. Zool_. lxi. 1896, pp. 420-498; Campana,
_Physiologie de la respiration chez les oiseaux. Anatomie de
l'appareil pneumatique_ ... (Paris, 1875); A.H. Garrod, "Major
Divisions of Passerine Birds (syrinx, &c.)," _P.Z.S._, 1876, pp.
506-519; and "On the Conformation of the Thoracic Extremity of the
Trachea in the Class _Aves_," _P.Z.S._, 1879, pp. 357-380; J. Mueller,
_Stimmorgane der Passerinen_, Mueller's Arch. (1847); and _Abh. Akad.
Wiss_. (Berlin, 1845-1847), translation by F.J. Bell, Oxford, 1878; H.
Strasser, "Luftsacke der Voegel," _Morph. Jahrb_. iii., 1877, pp.
179-227; C. Wunderlich, "Unterer Kehlkopf der Voegel," _Nov. Act. Leop.
Carol_., 1884; Ph. C. Sappey, _Recherches sur l'appareil respiratoire
des oiseaux_ (Paris, 1847); W.A. Forbes, "Contributions to the Anatomy
of Passerine Birds (syrinx)," _P.Z.S._, 1880, pp. 380-386, 387-391;
1881, pp. 435-737; 1882, pp. 544-546, 569-571; W. Yarrell,
"Observations on the tracheae of Birds," _Trans. Linn. Soc_., 1827,
pp. 378-391.