68 THE STRUCTURE AND LIFE OF BIRDS chap. 



dition and recent diet of the bird. But, even when we 

 allow for inaccuracies due to this, the differences are 

 startling. 1 



The Heart and Circulation. 



Every part of the body is nourished by the blood. 

 Only through the agency of the blood can food and air 

 make good what is lost by wear and tear. 



The heart is a force-pump which drives the blood 

 to all parts of the body, and, when it returns impure 

 and loaded with used-up material, sends it to the 

 lungs to be purified, after which it is despatched all 

 over the body again. On the voyage much of it 

 passes through the kidneys, which help the lungs to 

 purge it of the waste of the tissues. The essentials of 

 an efficient heart are that it should be strong, and that 

 it should keep the pure blood separate from the impure. 

 These two essentials are found combined in the hearts 

 of mammals and birds. They are strong muscles : 

 that part at least of them which forces the blood 

 through the arteries is remarkable for its strong 

 thick walls. And, thanks to the perfection of the 

 machinery, the blood which has been purified in the 

 lungs is never mixed with the impure blood which is 

 coming from the body. 



The heart is divided into right and left chambers 

 by a division through which there are no doorways. 

 The right and left chambers are each divided into two, 



1 See Bronn's Thier-Reich, vol. "Aves," p. 68 1. The figures, 

 as I have quoted them, are very nearly exact. For simplicity I 

 have disregarded small fractions. 



