26 PHYSIOLOGY AND ANATOMY. 
heart, and are composed of blood and air-vessels. They are 
so light they would float in the water, They are sometimes 3 
called bellows, because they contain so much air. They ap- — 
pear Jike the branches of a tree, and extend each side of the — 
heart. When we take in a breath we inhale the air tochange — 
the color of, and to purify the blood. . ) Ss 
_ Air is all around ns, and though we can not see or handle 4 
it, yet it has been divided into several gasses, called oxygen, — 
carbon, and nitrogen. When the air is thus divided, one — 
part of it will sustain life, and the others are very injurious. — 
The oxygen of the air unites with the dark blood in the lungs, — 
- and turns it to a red color, which then rushes back into the — 
heart. The muscles of the heart contract, and send it out — 
through the vessels, called arteries, to make skin, bone, flesh, ~ 
and very part of the body. It finally terminates in the , 
pillavies and vein, when itis changed into. a. darby 
red color. Tt is then unfit again for nourishment, as some of 
~ the good qualities have been taken out in its circulation. — 
This dark blood then unites with the new blood formed from _ 
the chyle, is sent into the heart, thence to the lungs, and is 
purified by the air, then sent back with its color changed, — 
and proceeds as I have previously stated. It may be asked 
why the blood does not rush back again to the heart after it 
has entered the artery? Tt is because there are little valves _ 
or trap-doors that shut over the arteries when the blood has 
entered them. These move as easily &s a door closes on its 
hinges, and prevents the return of the blood.’ The arteries _ 
are larger than the veins, and lie deeper in the system, or 
more removed from the surface. They serye to carry the 
blood out of the heart, and to distribute the proper nourish- 
ment where it is required. The veins are more numerous 
