812 PHYSIOLOGY 



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-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 



FIG. 358. 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 enclose the corpuscles 

 in their meshes (Fig. 358). 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 fila- 

 ments 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 or four minutes the twigs can be withdrawn and the spongy fibrin collected.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 deter- 

 mined 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. 



