When blood is first drawn from the living body it is perfectly liquid, flowing in any direction as readily as water. This condition is only temporary; in a few minutes the blood becomes viscid and sticky, and comes to resemble a thick red syrup; the viscidity becomes more and more marked, until, after the lapse of five or six minutes, the whole mass sets into a jelly which adheres to the vessel containing it, so that this may be inverted without any blood whatever being spilled. This stage is known as that of gelatinization, and is also not permanent. In a few minutes the top of the jelly-like mass will be seen to be hollowed or "cupped," and in the concavity will be found a small quantity of nearly colorless liquid, the blood-serum. The jelly next shrinks so as to pull itself loose from the sides and bottom of the vessel containing it, and as it shrinks it squeezes out more and more serum. Ultimately we get a solid clot, colored red and smaller in size than the vessel in which the blood coagulated, but retaining its form, and floating in a quantity of pale yellow serum. The whole series of changes leading to this result is known as the coagulation or clotting of the blood.

How do the colorless corpuscles differ from the red in size and number? What is each? What property does it possess? What is seen when one is watched with the help of a microscope? What is pus? Why are the movements of the colorless corpuscles called amaeboid?

What is the consistency of fresh drawn blood? What change occurs in it within a few minutes?

Cause Of Coagulation

If a drop of fresh drawn blood be spread out and watched with a powerful microscope, it will be seen that its coagulation is due to the separation of very fine solid threads which run in every direction through the plasma and form a close network entangling all the corpuscles. These threads are composed of an albuminous substance known as fibrin. When they first form, the whole drop is much like a sponge soaked full of water (represented by the serum) and having solid bodies (the corpuscles) in its cavities. After the fibrin threads have been formed they begin to shorten; hence the fibrinous network tends to shrink in every direction, and this shrinkage is greater the longer the clotted blood is kept At first the threads stick too firmly to the bottom and sides of the vessel to be pulled away, and thus the first sign of the contraction of the fibrin is seen in the cupping of the surface of the gelatinized blood where the threads have no solid attachment, and there the contracting mass presses out from its meshes the first drops of serum. Finally the contraction of the fibrin overcomes its adhesion to the vessel, and the clot pulls itself loose on all sides, pressing out more and more serum. The great majority of the red corpuscles are held back in the meshes of the fibrin.

What is meant by the stage of gelatinization? What first follows that stage? What next? What is the final result? What is the whole process called?

What is seen on watching a drop of fresh drawn blood with the aid of a good microscope? What are the separated threads composed of? To what may we compare a drop of blood in the first formation of the fibrin threads? What do the threads do after their formation?

Whipped Blood

The essential point in coagulation being the formation of fibrin in the plasma, and blood only forming a certain amount of fibrin,* if this be removed as fast as it forms the remaining blood will not clot. The fibrin may be separated by what is known as " whipping" the blood. For this purpose fresh drawn blood is stirred up vigorously with a bunch of twigs or a bundle of wire, and the sticky fibrin threads as they form adhere to these. If the twigs be then withdrawn a quantity of stringy material will be found attached to them. This is at first colored red by adhering blood-corpuscles, but by washing in water pure fibrin may be obtained perfectly white and in the form of highly elastic threads. The blood from which the fibrin has been in this way removed looks just like ordinary blood, but has lost its power of coagulating spontaneously.

Uses Of Coagulation

The living circulating blood in the healthy blood-vessels does not clot; it contains no solid fibrin, but this forms in it, sooner or later, when the blood gets in any way out of the vessels or if the lining of these is injured. By the clotting the mouths of the small vessels opened in a wound are clogged up, and the bleeding, which would otherwise go on indefinitely, is stopped. So too, when a surgeon ties an artery, the tight ligature crushes or tears its delicate inner surface, and the blood clots where this is injured. The clot becomes more and more solid, and by the time the ligature is removed has formed a firm plug in the cut end of the artery, which prevents bleeding.

Why is the first sign of their contraction seen in the cupping ? What is the final result of this contraction ? Why is the clot red ?

How can we prevent blood from clotting? How is blood whipped ? What do we find on examining the twigs after whipping blood ? How may we get the pure fibrin ? What are its characters ? How does whipped blood differ from ordinary blood?

* Fibrin is formed from fibrinogen, a soluble albumen existing In blood-plasma.