THE SONG OF THE SKYLARK in 



labouring so hard. A man would find it enough to run 

 or climb, without attempting to sing as well. How does 

 the soaring lark manage to keep up his song until he is 

 lost in the blue sky ? Even that is too little for him ; 

 he loves to rise and fall again and again, singing all the 

 time. 



Part of the explanation (the easiest part) lies in the 

 structure of a bird's breathing organs. When a man 

 breathes, he expands the chest, and so draws -in a little 

 fresh air to mix with the much larger quantity of vitiated 

 air which the lungs already contain. By no effort can 

 we change more than about half of the air in the lung at 

 a single inspiration, and in ordinary breathing we do not 

 replace more than about one-seventh of the air. There 

 is no means of sweeping out the human lung with a through 

 draught. But in the bird all the air in the lung can be 

 changed at a single inspiration. The lungs are prolonged 

 into capacious air-sacs, of which there is no trace in man, 

 and these contain far more air than the lungs themselves. 

 Thus, when the chest of the bird expands, a large volume 

 of air is drawn through the comparatively small lung, and 

 the air-sacs are filled with almost pure air, which venti- 

 lates the lungs, both as it enters and as it leaves the body. 



It is a common thing in zoological laboratories to fill 

 the air-sacs of a bird with some stiffening liquid. Paraffin- 

 wax will do, or cocoa-butter, or plaster of Paris, or a fusible 

 metal with very low melting-point. The injection runs 

 far beyond the limits of the thorax, to the farther end of 

 the abdomen, and into the neck ; it even penetrates most 

 of the bones. Of course the injection will not run unless 

 a vent is made for the air to escape by. If the humerus, 

 or principal wing-bone, is broken across, the air will escape 

 that way. Such an injection gives convincing informa- 

 tion as to the great volume of the air-sacs, which fill a large 

 fraction of the body-space. These great air-receptacles 

 seem to act like the wind-bag of some old-fashioned musical 

 instruments, and can, without being refilled, keep a small 



