156 THE FLIGHT OF BIRDS 



separating the two on the right from the two on the 

 left, and the arterial blood is kept quite apart from 

 the venous. The valve between the upper and lower 

 chambers on the right 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 



