448 THE BLOOD [CH. XXIX. 



2. The addition of a large quantity of neutral salts such as sodium 



sulphate or magnesium sulphate. 



3. Addition of a soluble oxalate, fluoride, or citrate. 



4. Injection of commercial peptone (which consists chiefly of 



proteoses) into the circulation of the living animal. 



5. Addition of leech extract to the blood, or injection of leech 



extract into the circulation while the animal is alive. 



6. Contact with the living vascular walls. 



7. Contact with oil. 



The cause of the coagulation of the blood may be briefly stated 

 as follows : 



When blood is within the vessels, one of the constituents of the 

 plasma, a protein of the globulin class, called fibrinogen, exists in a 

 soluble form. When the blood is shed, the fibrinogen molecule is 

 altered in such a way that it gives rise to the comparatively insoluble 

 material fibrin. 



The statement has been made that the fibrinogen molecule is split into two 

 parts ; one part is a globulin (fibrino-globulin), which remains in solution ; the 

 other and larger part is the insoluble substance fibrin. It is, however, doubtful if 

 this really represents what occurs, for recent work seems to show that the fibrino- 

 globulin is not a product of fibrinogen, but exists in the blood-plasma beforehand. 

 At any rate, whether this is so or not, the fact remains that fibrin is the important 

 product and the only one which need concern us. 



The next question is, What causes the transformation of fibrinogen 

 into fibrin ? and the answer to that is, that the change is due to the 

 activity of an enzyme which is called fibrin-ferment or thrombin. 



This enzyme does not exist in healthy blood contained in healthy 

 blood-vessels, but is formed by the disintegration of the blood- 

 platelets and colourless corpuscles which occurs when the blood 

 leaves the blood-vessels or comes into contact with foreign matter. 

 Hence the blood does not coagulate during life. But it will be said, 

 disintegration of the blood- corpuscles occurs during life, why, then, 

 does the blood not coagulate? The reason is, that although the 

 formed elements do disintegrate in the living blood, such a 

 phenomenon takes place very slowly and gradually, so that there 

 can never, under normal circumstances, be any massive liberation 

 of fibrin-ferment, and further, that there are agencies at work to 

 neutralise the fibrin-ferment as it is formed. The most noteworthy 

 of these neutralising agencies is the presence in the blood of an 

 antiferment called antithrombin, analogous to the antipepsin and 

 antitrypsin which we shall see are efficacious in preventing the 

 stomach and intestines from undergoing self-digestion. 



Nucleo-proteins obtained from most of the cellular organs of 

 the body produce intravascular clotting when injected into the 

 circulation of a living animal. In certain diseased conditions intra- 

 vascular clotting or thrombosis sometimes occurs, and this, if it 



