THE CIRCULATION OF THE BLOOD. 297 
THE CIRCULATION OF THE BLOOD, 
Now, let us follow the blood as it comes rushing along from 
the various parts of the body, and pours itself into the right 
auricle of the heart. 
From this chamber it is forced down into 
the bottom chamber, on the same side, and is prevented from 
rushing back by a wonderful 
system of valves, which allow 
it to rush down, but not back 
again. From the ventricle, 
the heart-muscles pump the 
blood up into the lungs. 
Here it spreads out into 
smaller and smaller tubes, 
until they are so small they 
cannot be seen with the 
naked eye. This blood, 
which the heart has pumped 
into the lungs, is_ called 
Venous blood, because it is 
full of poison gathered up 
from all over the _ body. 
The body is _ constantly 
burning. The cells which 
compose it are dying in mil- 
lions every second of life. 
Their dead bodies fall into 
the blood and are carried 
away to the heart and 
pumped into the lungs. Now, 
when we take in a breath of 
fresh air, we breathe in a 
quantity of gas, known as 
oxygen. 
sacs. 
Fic. 123.—From the artery the blood runs 
into smaller and smaller channels, so 
small that they can only be seen with a 
strong microscope. They again run to- 
gether and pour their contents into a 
vein. (From Blackie’s Physiology.) 
The lungs contain millions of little hollow bags, known as 
Around these, the tiny blood vessels are embedded. They 
are gorged with blood. Their walls are very thin, so that when 
the air rushes into these tiny bags, the oxygen is sucked up into 
the blood through the thin walls of the blood vessels. 
It in- 
stantly mixes with the poisons in the blood, and a wonderful, 
