THE EVOLUTION OF THE BIRD 



the mechanism for sending a supply of oxygen to the 

 muscles is much better in the bird or mammal than in the 

 lizard. 



The heart of a bird consists of two pumps, placed side by 

 side. Into one of these pumps, that on the left side, blood 

 full of oxygen comes from the lungs. This blood is then 

 pumped forward through a great tube, which turns over to 

 the right side of the animal and gives off blood vessels to all 

 the muscles and all parts of the body except the lungs. All 

 this blood, after being deprived of its oxygen, goes back to 

 the right side of the heart and is then sent to the lungs to get 

 a new supply of oxygen. The most peculiar part of the whole 

 mechanism is that the great main vessel, the aorta, instead of 

 lying in the middle line of the body, where we should natu- 

 rally expect to find it, actually crosses from the left to the 

 right side. We can explain this anomaly at once when we 

 examine the structure of a crocodile. The heart of the croco- 

 dile is very like that of the bird, but in the crocodile there are 

 two great vessels, one coming from the left side of the heart 

 and the other from the right side. These cross one another in 

 the middle line of the body and then pass upward until they 

 join and form a single aorta. The vessel from the left side 

 persists in birds, because it conveys only oxygenated blood, 

 but that from the right side, although it is found in the very 

 young chick, is blocked up in the full-grown bird, because 

 it conveys impure blood. Thus the heart of the bird is bet- 

 ter than that of the crocodile in that it supplies to every 

 muscle the maximum quantity of oxygenated blood, and its 

 peculiarities are explained by the structure of the heart of 

 the crocodile. 



By changes of the kind described, the bird ancestor was 

 able to maintain prolonged and uniform activity. The first 

 use to which it would put this power would naturally be in 



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