ORIGIN AND CHARACTERISTICS 49 



on that particular side. The Trachea is a flexible 

 tube composed of a large series of rings, and 

 is the passage by means of which a bird not 

 only breathes but through which most of its 

 notes are uttered. The dilatation of a part of 

 the trachea, usually near the middle, is a feature 

 found in many groups of birds, whilst some of 

 the Anseres even present a second expansion, 

 either close to the larynx or more generally near 

 the lower end. Another remarkable tracheal 

 modification exists in the males of most Anseres, 

 some half-dozen or more of the lowest rings 

 being fused together, and forming what is tech- 

 nically known as the bulba ossea or '^ laby- 

 rinth." In some species of Swans and Cranes the 

 trachea penetrates into the keel of the sternum; 

 the excessive lengthening of the trachea causes 

 it in some birds to be looped. These various 

 modifications exert considerable influence on the 

 cries of certain species, but the song owes its 

 production to another modification at the lower 

 end of the trachea and the adjoining parts of 

 the bronchii. This modification is termed the 

 Syrinx, and is absolutely peculiar to birds. It 

 consists of a series of muscles attached to the 

 extremities of the bronchial semi-rings. These 

 voice muscles reach their highest development 



D 



