[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This clotting of the blood is due to the formation in the blood, after it is withdrawn from the living body, of a substance called fibrin.[31] It is made up of a network of fine white threads, running in every direction through the plasma, and is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 87.  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.