I3§ THE STRUCTURE AND LIFE OF BIRDS chap. 



the two bronchi leading to either lung, a bony enlarge- 

 ment ■ is found, formed partly from the lower rings 

 of the windpipe, partly from the upper ones of 

 the bronchi. The latter on the inner side are of 

 membrane only. A bar of bone, the pessulus (P in 

 fig. 34 B), formed where the sides of the two bronchi 

 meet, passes across the syrinx from front to back. 

 From this bar a membrane, scalloped like a half-moon 

 on its outer edge, the membrana semilunaris (M.S. in 

 fig. 34 B), extends some way across the mouth cf the 

 bronchus. Opposite to it from the outer wall of the 

 syrinx projects another membrane. On the other 

 side of the pessulus is a similar crescent-shaped mem- 

 brane with another facing it. Thus, there are two 

 pairs of membranes, and there are muscles which can 

 tighten each pair and make the edges parallel. Many 

 birds have only two pairs of muscles for this purpose, 

 one pair passing to the trachea from the clavicles, the 

 other from the breastbone. But the majority of them 

 have at least one additional pair of syrinx muscles, 

 some as many as seven pairs, all having both points 

 of attachment on the trachea. Long vocal chords I 

 make a low voice, short ones a high voice. Hence 

 treble notes are characteristic of most birds and other I 

 small creatures. By tightening the chords the tone is 

 raised, by relaxing them it is lowered. The fact that 

 birds have so little range of voice seems to show that 

 the tension does not vary very much. The harsh, gruff 

 note of the Nightingale, and the abortive attempts of 

 the Cuckoo, when their vocal time is past, may be due 

 to the relaxation of the chords. 



The chief difference between the syrinx of a songster 



