after being further separated from the nourishing matter which it
contains, is thrown out of the body. There are multitudes of small
vessels, called _lacteals_, which, as these two mixed substances pass
through the long and winding folds of the intestines in the abdomen,
absorb the chyle, and convey it to the _thoracic duct_, which runs up
close by the spine, and carries the chyle, thus received, into a branch
of the _vena cava superior_, at _t_, whence it is mingled with the blood
going into the heart. In this engraving, the _lacteals_ and _thoracic
duct_ are not shown; but their position is indicated by the dotted
lines, marked X, Y; X, being the lacteals, and Y, the thoracic duct.

In the upper half of the engraving, H represents the heart; _a_, the
commencement of the _aorta_; _v c s_, the termination of the _vena cava
superior_. On each side of the heart, are the lungs; _l l_, being the
left lobe, and _r l_, the right lobe. They are composed of a network of
air-vessels, blood-vessels, and nerves. W, represents the _trachea_, or
_windpipe_, through which, the air we breathe is conducted to the lungs.
It branches out into myriads of minute vessels, which are thus filled
with air every time we breathe. From the heart, run the _pulmonary
arteries_, marked _p a_. These enter the lungs and spread out along-side
of the branches of the air-vessels, so that every air-vessel has a small
artery running side by side with it. When the two _vena cavas_ empty the
blood into the heart, the latter contracts, and sends this blood,
through these pulmonary arteries, into the lungs.

As the air and blood meander, side by side, through the lungs, the
superabundant carbon and hydrogen of the blood combine with the oxygen
of the air, forming carbonic acid gas, and water, which are thrown out
of the lungs at every expiration. This is the process by which the chyle
is converted into arterial blood, and the venous blood purified of its
excess of carbon and hydrogen. When the blood is thus prepared, in the
lungs, for its duties, it is received by the small _pulmonary veins_,
which gradually unite, and bring the blood back to the heart, through
the large _pulmonary veins_, marked _p v_, _p v_.

On receiving this purified blood from the lungs, the heart contracts,
and sends it out again, through the _aorta_, to all parts of the body.
It then makes another circuit through every part, ministering to the
wants of all, and is afterwards again br