176 The Bird 



are most numerous, but there are many exceptions. The 

 bones of storks and vultures (birds of great powers of 

 flight) are extremely pneumatic, while the bones of the 

 flightless ostriches are filled with marrow, and in the 

 aquatic penguins even this is reduced to a thread, the 

 bones being almost wholly osseous tissue. A swan, 

 although a heavy bird, flies remarkably well when once on 

 the wing, and is highly aerated, but, on the other hand, 

 terns and swifts — past-masters both in aerial evolutions 

 — have solid bones! 



Now an athlete who is trained in running has always 

 a very large lung capacity. Two persons of equal health 

 and strength, one of whom has run many races or who 

 has the power of keeping up a dog-trot for hour after hour, 

 while the other has led a more sedentary life, may show 

 a remarkable difference in the amount of air which they 

 can draw into their lungs— perhaps one hundred or one 

 hundred and fifty cubic inches more in the case of the 

 runner. The average person uses only about one sev- 

 enth of his lung capacity in ordinary breathing, the rest 

 of the air remaining at the bottom of the lung, being 

 termed "residual." As this is vitiated by its stay in the 

 lung, it does harm rather than good by its presence. When 

 great exertion is required, as in running, the person w^ho 

 can admit the largest amount of fresh air to his lungs in 

 each breath has command of an equally great power 

 of action. 



As we have seen, the lungs of a bird are small and 

 non-elastic, but this is more than compensated by the 

 continuous passage of fresh air, passing not only into 



