THE BLOOD 917 



have the power of ingesting granules of carmine, food, or dead bacteria 

 with which they may come in contact. 



In addition to these two classes, a third body is generally described 

 under the name of ' blood-platelets ' or heematoblasts. These are 

 especially well seen when the blood has been received directly into an 

 excess of osmic acid. It is still doubtful whether they are pre- 

 existent in the circulating blood or are formed in the plasma by a 

 process of precipitation. 



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 blood- 



FIG. 354A. 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.) 



vessels. If blood be received into a beaker it is at first perfectly fluid, 

 so that it can be poured from one vessel 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 a 

 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 fibrin, which 

 have formed throughout the plasma and enclose the corpuscles in their 

 meshes (Fig. 354A). On leaving the clot for some hours, drops of yellow 

 fluid appear on its surface and run together. The whole clot contracts, 

 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 four minutes the twigs can be withdrawn and the spongy fibrin 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 undergoes 

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

 matrix, formed elements or cells of various kinds. 



