120 THE ANATOMY OF BIRD-SONG. 



largely and peculiarly developed, and serve 

 a good turn in the matter of song-making, 

 each inflated membrane, especially those of 

 the anterior part of the thorax, acting as a 

 drum-head or sounding-board. In a word, 

 the whole bird is pneumatic, and often in 

 the ecstasy of great song-bursts, it vibrates 

 throughout from tip to tip, as if shaking the 

 sound in a spray of melody from its shining 

 feathers. The peculiar swaying and palpitat- 

 ing motions with which most birds accom- 

 pany their singing are attendant upon the 

 respiratory exertions necessary to keep the 

 pneumatic sacs full of air. Here again that 

 semi-automatic action, peculiar to the avian 

 structure, comes into play, for each one 

 of these respiratory movements affects the 

 trachea, larynx, and glottis by muscular 

 interference, if I may so call it. That 

 is, muscles called into action for one pur- 

 pose affect other muscles, and cause them 

 to contract, thus bringing about an in- 

 voluntary double movement. Thus when 

 the barn-yard cock stretches his neck at full 

 length to crow, the movement depresses the 

 glottis, lifts the tongue, and forms a sound- 

 chamber in the posterior mouth-cavity, all 

 at once. 



The lips of the mocking-bird's glottis are 

 extremely elastic, and may be so drawn as 

 to give any shape to the glottidian fissure that 

 may be desired in forming a note. So when 

 the tongue is drawn back with the barbs or 

 prongs of its bony tip close above the glottis, 

 and by muscular action is set quivering, the 

 purest flute notes are disintegrated and sent 

 forth in a shower of sounds, pleasing or harsh 

 according to the correlation of the other fac- 

 tors of modification. 



I take the mocking-bird {minius poly glottis) 

 as the highest example, in my discussion of 



