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 naturally 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 crocodile 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 better 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
[ 245 ]
