I CLASSIFICATION 13 
the trachea or windpipe in supplying air to the lungs, its forma- 
tion is worthy of attention. Its upper end consists of the 
larynx, and it passes down the neck as a flexible tube, formed 
by a continuous succession of bony rings connected by membrane, 
until it bifureates into two bronchi, which open into the lungs. 
A common feature, found in many groups not nearly allied, is 
the dilatation of a portion, generally near the middle, while a 
remarkable modification is exhibited by the males of many of the 
Duck-tribe, some of the lowest rings being fused together and 
forming what is known as the bulla ossea or “labyrinth.” In 
other Anatidae (some of the Swans), and some of the Cranes, the 
trachea enters the keel of the sternum; but a not unfrequent 
modification, usually confined to the male sex, often occurs else- 
where, when the windpipe is looped back upon itself. All these 
arrangements, however they may affect the sounds uttered by 
Birds, do not in themselves constitute the voice organ of most. 
That is reserved for the syrinxz, a pecularity of the Class Aves, 
consisting of the lower end of the trachea and the adjoining part 
of the bronchial tubes; and the varied modulations are effected 
by means of muscles attached thereto. These voice-muscles may 
be wholly absent or of the simplest character, but they attain their 
highest perfection in the Passeres, and especially in the large 
group of them known as Oscines, where there are often five or 
seven pairs. In this group the lowest four or five tracheal 
rings are solidly fused into a little bony box communicat- 
ing with the bronchi; the first and second bronchial rings (or 
in this part often semi-rings) being closely attached to the 
trachea, and the spaces between the second and third and the 
third and fourth being generally closed by an outer typaniform 
(drum-like) membrane, while the rest of the semi-rings of the 
bronchi are closed by the inner tympaniform membrane. It 
should be clearly understood that all the notes emitted by Birds 
are produced by the above structures only, and that the tongue has 
nothing to do with their utterance, except, possibly, in the case 
of the sounds that Parrots (but not other birds) are taught to 
produce. 
Classification.—The Classification of Birds is still in a con- 
dition of uncertainty, notwithstanding the many schemes succes- 
sively propounded during more than two centuries. To dwell 
upon them here would be impossible, and it is only practicable 
