7. Put two or three drops of fresh blood on a white
individual butter plate inverted in a saucer of water. Cover it with an
inverted goblet. Take off the cover in five minutes, and the drop has
set into a jelly-like mass. Take it off in half an hour, and a little
clot will be seen in the watery serum.

Experiment 88. _To show the blood-clot._ Carry to the slaughter
house a clean, six or eight ounce, wide-mouthed bottle. Fill it with
fresh blood. Carry it home with great care, and let it stand over night.
The next day the clot will be seen floating in the nearly colorless
serum.

Experiment 89. Obtain a pint of fresh blood; put it into a bowl,
and whip it briskly for five minutes, with a bunch of dry twigs. Fine
white threads of fibrin collect on the twigs, the blood remaining fluid.
This is "whipped" or defibrinated blood, which has lost the power of
coagulating spontaneously.

183. General Plan of Circulation. All the tissues of the body depend
upon the blood for their nourishment. It is evident then that this vital
fluid must be continually renewed, else it would speedily lose all of its
life-giving material. Some provision, then, is necessary not only to have
the blood renewed in quantity and quality, but also to enable it to carry
away impurities.

So we must have an apparatus of circulation. We need first a central
pump from which branch off large pipes, which divide into smaller and
smaller branches until they reach the remotest tissues. Through these
pipes the blood must be pumped and distributed to the whole body. Then we
must have a set of return pipes by which the blood, after it has carried
nourishment to the tissues, and received waste matters from them, shall be
brought back to the central pumping station, to be used again. We must
have also some apparatus to purify the blood from the waste matter it has
collected.

[Illustration: Fig. 68.--Anterior View of the Heart.

A, superior vena cava;
B, right auricle;
C, right ventricle;
D, left ventricle;
E, left auricle;
F, pulmonary vein;
H, pulmonary artery;
K, aorta;
L, right subclavian artery;
M, right common carotid artery;
N, left common carotid artery.
]

This central pump is the heart. The pipes leading from it and
gradually growing smaller and smaller are the arteries. The very
minute vessels into which they are at last subdivided are
capillaries. The pipes which convey the blood back to th