BLOOD CLOTTING 105 



that the calcium acts ; for example, clotting occurs when purified throm- 

 bin is added to dialyzed oxalate blood or plasma or to a pure solution of 

 fibrinogen. Citrates prevent clotting by forming calcium citrate, which 

 although soluble does not ionize in solution. It is the free calcium ions 

 that are important. The action of the fluoride is somewhat mysterious, 

 for it has been found that to produce clotting in fluoride plasma the sim- 

 ple addition of calcium chloride will not suffice; thrombin itself must be 

 added as well. Some authors assert, however, that if the calcium chlo- 

 ride is added cautiously to " fluoride" blood, it will induce clotting 

 (Rettger). In any case it appears that the fluoride does something more 

 than precipitate the calcium; possibly it prevents the breaking up of 

 platelets and leucocytes. 



The Influence of the Tissues. As already stated, when slowly clotting 

 blood, like that of a bird, is collected through a sterile glass tube into a 

 thoroughly clean vessel and immediately centrifuged, the plasma will 

 often remain indefinitely unclotted. If an extract of some tissue, such 

 as muscle, is added, however, the plasma immediately clots. To a much 

 less degree, the same phenomenon is exhibited by mammalian plasma 

 when it is collected in a similar manner. From these observations the 

 conclusions may be drawn that the tissues furnish some substance as- 

 sisting in the clotting process, and that this substance is also formed 

 from certain elements present in mammalian but not present in avian 

 blood. The absence of platelets from the latter blood suggests that 

 these must be the source of the activating substance in mammalian blood. 

 It is plain that this tissue factor in clotting is of importance in hasten- 

 ing the process when an animal is wounded. 



Before attempting to formulate an hypothesis that will explain the 

 process of clotting, it is necessary to call attention to one other impor- 

 tant fact. This refers to the presence in blood of a substance that pre- 

 vents clotting and is hence called antithrombin. Antithrombin is pres- 

 ent in normal blood, for a given specimen of pure fibrinogen will clot 

 less rapidly when mixed with serum to which some oxalated plasma has 

 been added than with an equal amount of the same serum correspond- 

 ingly diluted with a solution of soluble oxalate. A striking increase 

 in the concentration of antithrombin in blood can be brought about by 

 rapidly injecting a solution of commercial peptone into the blood ves- 

 sels fifteen to thirty minutes before bleeding. The peptonized blood or 

 plasma will remain fluid for many hours, if not indefinitely. That the 

 failure of this blood to clot depends on the presence of some anticlotting 

 substance, and not upon the absence of one of the necessary clotting sub- 

 stances (fibrinogen, thrombin, etc.), is evidenced by the fact that the 

 addition of some of it to a mixture of thrombin and fibrinogen inhibits 



