84 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



6. Injection of nucleo-proteid produces intravascular clotting. 

 Very minute doses, however, produce the opposite efifect^ — namely 

 delay of coagulation. 



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. Contact with the living vascular walls. 



4. Contact with oil. 



5. Addition of a soluble oxalate (e.g. potassium oxalate) : this 

 precipitates the calcium necessary for coagulation as insoluble calcium 

 oxalate. 



6. Injection of commercial peptone (which consists chiefly of 

 proteoses) into the circulation of the living animal. 



7. Addition of leech extract. This acts in virtue of a proteose it 

 contains. 



The theory generally received which accounts best for the coagu- 

 lation of the blood is that of Hammarsten, and it may be briefly 

 stated as follows : — 



When blood is loithin the vessels one of the constituents of the plasma, 

 a proteid of the globulin class called fibrinogen, exists in a soluble 

 form. 



When the blood is shed the fibrinogen molecule is split into two 

 parts : one part is a globulin, which remains in solution, the other is, 

 the insoluble material fibrin. 



This change is brought about by the activity of a special un- 

 organised ferment called the fibrin ferment. 



This ferment does not exist in healthy blood contained in healthy 

 blood vessels, but is otie of the products of the disintegration of the 

 white corpuscles and blood tablets that occurs when the blood leaves 

 the vessels or comes into contact with foreign matter. 



To this may be added, as the result of recent research, that a 

 soluble calcium salt is essential for the formation of the ferment ; that 

 the fibrin ferment belongs to the class of nucleo-proteids ; that other 

 nucleo-proteids (Wooldridge's tissue-fibrinogens, see p. 29) obtained 

 from most of the cellular organs of the body produce intravascular 

 clotting when injected into the circulation of a living animal. 



The fibrin fei'ment may very conveniently be called thrombin. 

 Like other ferments it is preceded by a mother substance or zymogen 

 which may be called prothrombin ; the action of calcium salts is to 



