138 THE STRUCTURE AND LIFE OF BIRDS cuap. 
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 of 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 
make a low voice, short ones a high voice. Hence 
treble notes are characteristic of most birds and other 
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 
