68 THE STRUCTURE AND LIFE OF BIRDS cwap. 
dition and recent diet of the bird. But, even when we 
allow for inaccuracies due to this, the differences are 
startling 
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. 681. The figures, 
as I have quoted them, are very nearly exact. For simplicity I 
have disregarded small fractions. 
