20 
SECOND BOOK. 
from other creatures. Birds have a heart with four 
chambers, but the rest of the backboned animals 
have not, as you will learn very soon.” 
11. “ It is easy to understand about the milk food, 
the hair, and the limbs,” said William, “because 
The Human Heart (Mammal), opened 
to show the four chambers. 
Diagrammatic Form of Heart. 
we can use our eyes and see for ourselves, but the 
lungs and heart are, of course, hidden.” 
12. “ Perhaps by feeling you can find out what 
I want you to learn about the blood,” said Mr. 
Johnson. “ Place your hands on this lizard; now 
on this frog. Next press your hand on Rover, and 
lastly on your own body. Do you notice any differ- 
ence?” 
13 . The boys said that Rover’s body and their own 
felt quite warm, and that the lizard and frog did 
not. 
“ That is because you and Rover, who belong to 
