104 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



(positive phase). Very minute doses, however, produce the opposite 

 effect : namely, delay of coagulation (negative effect). 

 Coagulation is hindered or prevented by 



1. A low temperature. In a vessel cooled by ice, coagulation may 

 be prevented for an hour or more. 



2. The addition of a large quantity of neutral salts, like 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 (ribrinoglobulin), 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 fibrinoglobulin is 

 not a product of fibrinogen, but exists in the blood plasma before- 

 hand. 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 a special unorganised ferment which is called fibrin ferment 

 or throtnbin. 



This ferment 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, disintegra- 

 tion 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 



