156 THE FLIGHT OF BIRDS 
separating the two on the right from the two on the 
left, and the arterial Mod is kept quite apart from 
the venous. The valve between the upper and lower 
chambers on theright is a single flap of muscle, very 
unlike the three flaps of membrane found in the 
mammal’s heart. But it works in the same way, and 
is no less efficient. The blood swarms with red 
corpuscles, not round and without nucleus like ours, 
but oval and nucleated. Where they come from 
is a question. The marrow in human bones is 
believed to be a factory of red corpuscles, and so we 
cannot help wondering what substitute for this those 
birds may have whose chief bones are hollow, with 
only the very thinnest lining of marrow We know 
that they have somewhere as good a factory as any 
mammal has; the richness in corpuscles banishes 
all doubt on this point. It may be that the spleen 
is very active. It is known that in mammals during 
the embryo stages and in after-life, in emergencies, 
the spleen gives birth to many red corpuscles. 
Whatever their origin may be, there they are in their 
thousands, putting vitality and energy into the bird. 
The breathing apparatus is as wonderful as any 
part of this living flying machine. The lungs are 
very small and may be seen, brilliantly scarlet, 
neatly packed on either side of the backbone. Their 
efficiency is not to be measured by their very diminu- 
tive size. They have large extensions called air-sacs, 
into which the air rushes, passing through the lungs, 
when inspiration takes place. Some of the air 
breathed in finds its way at once into the minute 
air-passages that ramify about the lungs and does the 
work of oxidising the blood. The rest rushes straight 
