62 



STRUCTURE AND CLASSIFICATION OF BIRDS 



or bony bar, which runs across the base of the trachea, 

 arising from the last tracheal ring anteriorly, and attached 

 to the penultimate or antepenultimate ring posteriorly. 

 This bar is called the pessulus. It is shown in fig. 41, 

 which represents a syrinx seen from below. When the 

 syrinx is cut open it may be seen that this bony bar bears a 

 tough semilunar membrane, directed upwards between the 

 bronchi ; the voice is due to the vibrations of this mem- 

 brana semilunaris. 



Another external feature of the typical tracheo-bronchial 

 syrinx is the presence of a pair of mus- 

 cles which arise some way up the trachea 

 and are inserted on to an early semi- 

 ring of the bronchial series, one on each 

 side, or even to the last or nearly the last 

 of the tracheal series. From this starting 

 point we can follow the modification of 



FIG. 41. SYRINX OF Cymbirlu/uclt/ts, FIG. 42. SYRINX OF Balceniceps. 

 < 5, FROM BEHIND (AFTER FoKT.Ks). ', ligamentous rudiment of intrinsic 



muscle. 



the syrinx in a number of directions in the way of com- 

 plication, in the way of simplification, and in a direction of 

 alteration which can hardly be termed either complication or 

 simplification. 



The first sign of simplification is the disappearance of 

 the intrinsic syringeal muscles, which in many forms have 

 completely disappeared. The disappearance is not neces- 

 sarily associated with any other changes in the general 

 structure of the organ. Occasionally, as in Bal&niceps (see 



