Syrinx In Birds: Detailed Explanation With Diagrams

 

Syrinx In Birds: Detailed Explanation With Diagrams 

Syrinx or voice box or lower larynx ( as it is placed much lower compared to our organ of voice), is the bird's sound-producing organ. The structure called "Syrinx" is peculiar to the Class of Birds. The myriad of sounds that make up bird song are produced primarily by the syrinx, a small chamber at the bifurcation of the trachea.

    The syrinx is singularly uniform among birds making a great variety of vocal sounds. Among living animals, the syrinx is one of the unique characteristics of Aves providing uniqueness to the respiratory system of birds (e.g. pigeon) as are provided by the feathers.

Syrinx: An overview 

Syrinx means twin or double flute in Greek

The syrinx is located at the point where the trachea divides to form the two bronchi.

The position and structure of this organ vary within narrow limits, but in general, it is composed of several modified rings of the lower trachea or upper bronchial tubes.

Diagram of External view of Syrinx 

 It is formed by the modification of either the tracheal rings, the bronchial half rings or a combination of both. This can lead to different types i.e

• tracheal syrinx,
• tracheobronchial syrinx ( most common),
• or two separate bronchial syrinxes.

Anatomy of syrinx:

The mucus membrane of the syrinx constitutes a pad-like thickening and is provided with several muscles and membranes.

It has complicated structural construction.

L.S. of Syrinx: Diagram 

A bar of cartilage called the pessulus is present at the junction of two bronchi. It extends dorsoventrally inside the tympanum and holds a small fold of mucus membrane called the crescentic or membrana semilunaris.

The inner membraneous lining of the bronchi produces inconspicuous internal tympaniform membranes.

How sound is produced by Syrinx?

  

The sound is produced by the vibration of the membrana semilunaris while the pitch of the sound is controlled by the action of the syringeal musculature which includes: 

• a pair of intrinsic Syringeal muscles, arising from the lateral sides of the trachea and is attached to the syrinx and,
•  a pair of sterno-tracheal muscles which originate from the sternum and are inserted into the trachea.

An increase in the tension of the tympaniform membranes increases their frequency of vibration, and thus raises the pitch of the sound; an increase in the diameter of the passage way increases the volume of the sound (loudness).

 

Note: The semilunar membrane and the cartilaginous pessulus occur only in the syrinx of songbirds, probably contributing to the advanced singing abilities of these birds.

Schematic diagram of L.S. of Oscine Syrinx to show the mechanism of sound production 

The muscles stretch the two bronchi upward into the trachea (shown by large solid arrows) and twist the third bronchial cartilage. The latter pushes the lateral labia into the path of air flowing out of the respiratory system and into proximity with the medial labia, which have also been forced into the airway by the movement of the bronchi ( shown by medium solid arrows). 

Sound is produced as air rushes between the lateral and medial labia (shown by dashed arrows), causing these soft tissues to vibrate.

The position of the syrinx can be changed by the action of these muscles.

Major muscles of syrinx 

Diagram of front and side view of Syrinx in Birds to show the different muscles attached 

Tracheo-lateralis: Inferior bifurcations of the lateral tracheal muscle going to the third bronchial half-ring.

Broncho-trachealis brevis: inserted into the posterior end of the second bronchial half-ring.

Bronchialis posticus: posterior common laryngeal muscle, going from the last tracheal ring to hinder the end of the second bronchial half-ring.

Bronchialis anticus: anterior ditto, from last tracheal ring to fore ends of first and second bronchial half-rings.

Sterno-trachealis (sterno tracheal muscle): relaxor of tympaniform membrane, going from the side of the trachea to the sternum.

Intrinsic syringeal muscles:

Arising from the lateral sides of the trachea and is attached to the syrinx.

Studies on the structure and function of the syrinx have shown that the sound can be produced in three major ways, depending on how the syrinx is structured in the species and on how the bird can modify the airflow and overall shape of the syrinx using muscles attached to the outside walls of the syrinx.

• The external tympaniform membranes can vibrate like a drum to produce sound.
• Or the semilunar membrane and bony pessulus may act with external muscles of the syrinx to construct the openings of each bronchus, producing sound as air rushes from the lungs under pressure.
• Or the left and right tympaniform membranes may constrict to close down the primary bronchus on each side, producing a flute-like double note as seen in thrushes ( Family Turdidae), known for their ability to sing harmonious double notes simultaneously.

There may be no "membrana semilunaris" and only three pairs of additional muscles, in birds highly gifted as to their power of emitting special sounds.

A syrinx may be formed by the trachea only ( without the intervention of the bronchi) as in Thamnophilus, where the lower part of the trachea has delicate walls, and is flattened dorso-ventrally into six or seven delicate segments of rings, the rings being interrupted laterally.

A syrinx may be formed in each bronchus ( without the intervention of the trachea), as in Steatornis, where more than ten rings in each bronchus may be counted before reaching the syrinx, and where a pair of muscles passes from each bronchus to the trachea.


 The intrinsic muscles of the voice organ may be inserted into the ends of the bronchial semi-rings, or in what is called an Acromyoidal manner or into their middle parts, a mode which is distinguished as Mesomyoidal. A condition in which the trachea alone forms the vocal organ is spoken of as "Tracheophonal". An arrangement in which the lower end of the trachea is not modified to form a vocal organ is called Oligomyoidal. 

Parrots have no os transversale or septum dividing the lower end of the trachea, and they have only three pairs of intrinsic muscles. They speak with the aid of their tongue and beak alone. 

The majority of birds have a syrinx that contains elements of the right and left bronchi and thus have a right and left paired structures.

In oscines and some other birds, these paired structures include,  at the cranial end of each bronchus, a distinct set of membranes that vibrate to produce sound (as shown in the above diagram). The possibility thus arises that these birds could produce two harmonically unrelated tones at the same time; as if duetting with themselves.

This "two-voice" phenomenon has been studied in many species with surprising results.

Let's know a bit more 

The syrinx may be altogether absent, as in the American Vultures and the Ostrich and its allies.
 

Although the syrinx is the structure responsible for producing sound, there can be modifications in the trachea, made resonating or given a reverberating quality by a special structure or by windings of the trachea, before it reaches the syrinx.

• Many species of ducks have enlarged box bones, like a drum on the lower portion of the trachea, giving power to the bird's voice.
• Cranes and swans have veritable French horns on their breastbone.
• In the majestic Whooping Crane, the windings of the trachea reach their maximum. The entire windpipe which is about 4 ft in length, is one-half coiled within the sternum, or breastbone, giving remarkable volume and resonance to the voice.