THE LIVING BIRD 11 



feathers themselves, made of material that is itself 

 quite light — keratin — are built (like the famous mast 

 of the Enterprise) on the hollow cylinder principle, 

 combining lightness with strength. The bones 

 show the same plan and are individually lighter 

 than corresponding bones of equal size in either 

 reptile or mammal. Not only are they hollow but 

 air passages, directly connected with the lungs, pass 

 into many of them. The curious tendency for 

 various bones to fuse with each other, a character- 

 istically avian trait, goes to increased rigidity, as 

 important as lightness and strength. But a bird's 

 respiratory mechanism is perhaps its most wonder- 

 ful adaptation to life in the air. Flying is strenuous 

 exercise — some birds, e.g. various swifts, may 

 achieve a speed of more than 200 miles an hour — the 

 tissues demanding a constant and rapid supply of 

 oxygen via the blood stream. The rather solid and 

 heavy appearance of birds' lungs does not look 

 promising when one opens the body, but micro- 

 scopic examination reveals the structure as a vast 

 series of ramifying tubules, all intermingling with 

 one another and together producing an enormous 

 area of respiratory surface. Connected with the 

 lungs is a series of air passages and sacs traversing 

 most parts of the body and as has just been ex- 

 plained, extending actually into many of the larger 



