i8o The Bird 



seem to us. In changing, Nature seems to have tried 

 numberless experiments, only a few of which have sur- 

 vived. For example, we know that fish breathe by a 

 sort of swallowing, the water being taken in at the mouth 

 and poured out through the gill-clefts. So in frogs and 

 salamanders we find that, although the\' possess lungs, 

 yet they still employ a swallowing process to get the air 

 down their throats. This is the reason why a frog will 

 suffocate if its mouth is held open. There are certain 

 salamanders which are wholly without lungs, their moist 

 skin being so vascular that the blood is purified through 

 it. But strange to say, these amphibians still swallow and 

 swallow, as did their ancestors, although no air passes 

 down their throats, and indeed there is no place to which 

 it could go! As we have seen elsewhere, birds exhale 

 air largeh' b}' the action of certain abdominal muscles. 

 Watch a goldfish rise to the top of the water and eject or 

 gulp down a bubble of air, and observe the rapid breath- 

 ing of a bird, and you have the two extremes before you 

 — the swim-bladder of ages ago and the wonderful lungs 

 of a bird of to-day. 



The Heart and the Life-blood 



Perhaps the most wonderful organ in a bird's body 

 is its heart. In the very lowest of back-boned animals 

 the heart is merely a long tube, in fact a simple artery 

 or vein, which contracts at certain intervals and so pro- 

 pels in a forward direction the fluid which it contains. 

 A fish may almost be said to have its heart in its head, 

 so far forward in its bod}^ is it placed; nevertheless, as 



