THE ANATOMY OF BIRDS. — PNEUMATOLOGY. 
205 
appreciable by the rapt listener to the singularly varied kind and quality of notes trilled forth 
in the stillness of gloom by the nightingale." 
I should be able to make the plan of the syrinx clear to the student with the assistance of 
Macgillivray's beautiful figures. These are drawn from the rook, — a corvine croaker, indeed, 
but one whose syrinx is in good order, though he has never learned to play. As the modifica- 
tions affect principally the soft parts covering and moving the music-box, one description of the 
latter is applicable to most birds. The last lower ring, or piece composed of several fused rings, 
of the trachea, at its bifurcation into bronchi, is enlarged or otherwise modified (fig. 101, 
aba), and crossed below from front to back by a bony bar, the pessulus at b; a), or 
bolt-bar, which, dividing it into lateral halves (as at i^)^ forms thus two lateral openings 
instead of one median tube, — the beginnings of each bronchial tube. A membranous plate, 
strengthened by cartilage, rises vertically into the tracheal tube, forming a septum, or median 
partition, between the orifices of each bronchus. The free curved upper margin of this septum, 
extending of course, from front to back of the orifice, is called the semilunar membrane; being 
the edge of a partition common to both bronchi, it forms, in fact, the inner lip of each bronchial 
orifice ; that is to say, the inner rima glottidis syringis, or lip of the syringeal mouth-piece. 
This membrane vibrates with the column of air, and is, in fact, one of the ''vocal chords." 
Now the bronchial rings which succeed are not annular, circumscribing the bronchial tube, 
but are half-rings (}^, b, b), or arcs of circles to be completed by membrane, which forms more 
or less (scarcely or not half) of the circumference of the tube ; this membranous part, termed 
the internal tympaniform membrane (}^, c to c), being on the side of the bronchus which faces 
its fellow, while the hard bronchial half- rings complete the rest of the cylinder. The mem- 
brane is attached to the pessulus above. This accounts for the whole bronchial tube and its 
vocal septum from its fellow. Now the concavity of the upper two or three bronchial half- 
rings, on the outer wall of the tube, but in its interior, is the place where is developed a certain 
fold of the mucous membrane, projecting into the tube opposite the septum, and forming the 
outer lip of the syringeal glottis ; for this membranous fold, like the semilunar membrane, is 
set quivering in vocalization. The upper tracheal rings which enter into this arrangement 
are enlarged and otherwise modified. Thus are formed two ''vocal chords," upon the vibrations 
of which the harmonious or discordant notes of the bird depend. The cords are struck by the 
hand of air indeed, but endless musical variations result from the play of the muscles in increas- 
ing or diminishing and variously combining the tension of the several parts of tlie instrument. 
In giving four pairs of intrinsic syringeal muscles (anterior external, anterior internal, inter- 
mediate, and posterior, besides the extrinsic sterno-tracheales), as figured in 16, a, 6, c, d and e, 
Macgillivray is said to have understated the full oscine number, which is five or six. In the raven, 
Owen describes five, without counting the sterno-trachealis : broncho-trachealis anticus, anterior 
external; broncho-trachealis posticus, posterior external; broncho-trachealis brevis, posterior 
internal; fcroMcMaZis an^iats, anterior internal ; a,nd bronchialis posticus. The general arrange- 
ment, however, is fairly indicated by Macgillivray in where on the side of the syrinx, the mus- 
cles are seen to diverge from the tracheal lateral line to go to ends of the bronchial semi-rings. 
The student will understand that my description is particular only as regards the oscine 
syrinx ; that in birds at large every possible modification, almost, of lower tracheal and upper 
bronchial rings occurs, and with various musculation, or with none. The non-oscine rule for 
the muscles is, one on each side, if any ; and insertion into mid-parts, not ends, of the bronchial 
half-rings. The latter character chiefiy distinguishes the non-oscine syrinx when it has sev- 
eral muscles. As to situations of the syrinx, three have been recognized : the ordinary broncho- 
tracheal, in formation of which both bronchi and trachea take part ; the tracheal, only known 
to occur in some American Passeres, as in Thamnophilus and Opetiorhynchus, situated wholly 
in the trachea, the lower part of which is extensively membranous ; and the bronchial, wholly 
in the bronchi, as in Crotophaga and Steatornis. 
