112 THE ANATOMY OF BIRD-SONG. 



two clefts from the lungs sets these elastic 

 margins vibrating, and thus gives rise to a 

 musical note, the character of which is chiefly 

 determined by the tension of the elastic mar- 

 gins and the length of the tracheal column of 

 air. The muscles by the contraction of which 

 these two factors of the voice are modified, 

 are extrinsic and intrinsic." 



In the case of every true song bird that I 

 have examined there are six pairs of intrin- 

 sic muscles controlling the tympanum and 

 bronchial arcs. By the contraction of these 

 muscles, the arcs and the tympanum are 

 moved in a variety of ways. The mocking- 

 bird, however, has much weaker syrinx mus- 

 cles than the shripe, the cuckoo, or the blue- 

 jay, though if Prof. Huxley's theory of song- 

 generation is correct, the mocking-bird's 

 organ should be much the more powerful in 

 every respect. 



The "fold" of membrane described by 

 Prof. Huxley as occupying the inner surface 

 of the free arcs of each bronchus and the 

 membranous edge of the frame of the pessu- 

 lus are really delicate valves whose function 

 or office is quite different from that assigned 

 to them by the anatomists. 



The minute intrinsic muscles controlling the 

 mocking-bird's syringeal membranous pro- 

 jections are so arranged that the mouths of 

 the bronchial tubes may be almost, if not 

 quite, closed by the extension of the "folds," 

 or elastic margins, and by this means the 

 bird is enabled to measure in the nicest man- 

 ner the amount of air thrown from the lungs 

 into the trachea. Thus when it is whistling 

 a rapid staccato, the delicately modulated 

 notes following one another like raindrops in 

 a shower, these valves in the syrinx are doling 

 out the air in precisely the right quantities 



