THE COAGULATION OF THE BLOOD 953 



upon thrombogen in the presence of lime salts and produces thrombin. 

 By the intermediation of the thrombin the fibrinogen also present in 

 solution in the plasma is converted into fibrin. The changes occurring 

 in shed blood and resulting in the production of a clot are therefore 

 mainly concerned with the production of the fibrin ferment. This 

 view of the essential characters of coagulation is borne out by observa- 

 tions on other forms of plasma, especially of plasma obtained from 

 birds' blood. This when obtained with scrupulous cleanliness so as 

 to avoid any contamination with dust or with the tissues remains 

 permanently uncoagulable. In the plasma got by centrifuging the 

 blood no blood-platelets are to be seen, and no precipitate is produced 

 by exposure to a temperature of C. We may say therefore that 

 blood- platelets with their contained thrombokinase are absent from 

 birds' blood, and with them the property of spontaneous coagulability. 

 It is also free from fibrin ferment, but contains thrombogen as well as 

 soluble lime salts. It is only necessary therefore to add thrombokinase 

 in the shape of a watery extract of any tissue in order to cause the 

 appearance of fibrin ferment and the conversion of the fibrinogen 

 already present in the plasma into fibrin. 



In every case the initiation of the act of clotting would seem to 

 depend on the setting free of thrombokinase in the plasma. In 

 mammalian blood, although thrombokinase can be derived from red or 

 white corpuscles, we have no reason to believe that there is any 

 appreciable disintegration of these formed elements when the blood 

 leaves the vessels. In oxalate blood leucocytes can be seen alive and 

 exercising amoeboid movements two or three days after the blood has 

 left the vessels, and although certain observers have assumed the 

 presence of explosive corpuscles which break up directly the blood 

 leaves the vessels, the presence of such corpuscles in the higher animals 

 has not been demonstrated, though in some invertebrata they are 

 certainly present. The sole source therefore of the thrombokinase is 

 the very perishable formed elements of which we have spoken as the 

 blood-platelets. The very existence of this element is doubtful in 

 normal blood. What is certain is that any slight change in the plasma, 

 whether due to contact with a foreign object or to cooling of the blood, 

 causes the appearance of these elements. The first act therefore in 

 coagulation is the appearance of the blood-platelet and its disintegra- 

 tion with the setting free of thrombokinase. The part played by the 

 platelets in coagulation is of great importance in maintaining the 

 integrity of the vascular system. If a fine needle be thrust through 

 the wall of a venous capillary which is kept under observation by the 

 microscope it will be seen that the blood, as it flows past the injured 

 spot, deposits blood-platelets on the side of the puncture. These 

 aggregate to form a plug closely adherent to the wall of the vessel, which 



