lly shrinks, until at the end of a few hours
it is much firmer, and floats in the yellowish fluid. The white corpuscles
become entangled in the upper portion of clot, giving it a pale yellow
look on the top, known as the _buffy coat_. As the clot is attached to the
sides of the vessel, the shrinkage is more pronounced toward the center,
and thus the surface of the clot is hollowed or _cupped_, as it is called.
This remarkable process is known as coagulation, or the clotting of
blood; and the liquid which separates from the clot is called serum.
The serum is almost entirely free from corpuscles, these being entangled
in the fibrin.

[Illustration: Fig. 67.--Diagram of Clot with Buffy Coat.

A, serum;
B, cupped upper surface of clot;
C, white corpuscles in upper layer of clot;
D, lower portion of clot with red corpuscles.
]

This clotting of the blood is due to the formation in the blood, after it
is withdrawn from the living body, of a substance called fibrin.[31] It is
made up of a network of fine white threads, running in every direction
through the plasma, and is a proteid substance. The coagulation of the
blood may be retarded, and even prevented, by a temperature below 40
degrees F., or a temperature above 120 degrees F. The addition of common
salt also prevents coagulation. The clotting of the blood may be hastened
by free access to air, by contact with roughened surfaces, or by keeping
it at perfect rest.

This power of coagulation is of the most vital importance. But for this,
a very small cut might cause bleeding sufficient to empty the
blood-vessels, and death would speedily follow. In slight cuts, Nature
plugs up the wound with clots of blood, and thus prevents excessive
bleeding. The unfavorable effects of the want of clotting are illustrated
in some persons in whom bleeding from even the slightest wounds continues
till life is in danger. Such persons are called "bleeders," and surgeons
hesitate to perform on them any operation, however trivial, even the
extraction of a tooth being often followed by an alarming loss of blood.

Experiment 86. A few drops of fresh blood may be easily obtained to
illustrate important points in the physiology of blood, by tying a
string tight around the finger, and piercing it with a clean needle. The
blood runs freely, is red and opaque. Put two or three drops of fresh
blood on a sheet of white paper, and observe that it looks yellowish.

Experiment 8