THE BLOOD 855 



Unless special precautions are taken, the examination of blood obtained 

 from a blood vessel is interfered with by the process of clotting, which 

 ensues shortly after the blood has left the vessels. If blood be received 

 into a beaker it is at first perfectly fluid, so that it can be poured from 

 one beaker to another. After a space of time varying from three to eight 

 minutes it begins to be viscous, and if poured out of the beaker leaves 

 an adherent layer on the sides of the vessel. A minute later the whole 

 mass of the blood becomes solid and the beaker can be inverted without 

 spilling its contents. If section be made of this blood clot, it is found 

 to owe its solidity to a network of fine threads of a protein substance named 



FIG. 369. Network of fibrin, after washing away the corpuscles from a film of 

 blood that has been allowed to clot ; many of the filaments radiate from little 

 clumps of blood platelets. (SCHAFER.) 



fibrin, which have formed throughout the plasma and enclosed the corpuscles 

 in their meshes (Fig. 369). On leaving the clot for some hours, drops of 

 yellow fluid appear on its surface and run together. The whole clot con- 

 tracts, and finally there is a reduced clot floating or suspended in a yellowish 

 fluid known as serum. If after the blood has left the vessels it be whipped 

 with a bunch of twigs, or stirred with a glass rod, the filaments of fibrin as 

 they are formed are deposited on the twigs. After three or four minutes 

 the twigs can be withdrawn and the spongy fibrin collected. The blood 

 which is left consists only of the corpuscles plus serum, and will not clot, 

 since its fibrin has been removed. It is known as defibrinated blood. Since 

 the corpuscles are apparently unchanged in the meshes of the clot and 

 clotting can be produced in blood plasma entirely separated from corpuscles, 

 we must look upon the process of coagulation as determined in the main 

 by changes in the blood plasma. We can regard the blood therefore as a 

 tissue consisting of a fluid matrix, which is extremely unstable and under- 

 goes change when it leaves the vessels, and as having embedded in its matrix 

 formed elements or cells of various kinds. 



