MAN --AN ADAPTIVE MECHANISM 



body, awaiting the arrival of the special contingency 

 which provides the stimulus for their interaction. 

 Such are anti-bodies, the digestive agents and the 

 constituents which cause blood clotting, each of which 

 justifies its presence in the body by the specific pro- 

 tective activity it manifests in the face of the specific 

 contingency as a result of which it was evolved. 



Mechanism of Blood Coagulation 



The mechanism of blood coagulation is of vital 

 interest in this connection. The blood must be main- 

 tained in perfect fluid form within the uninjured 

 vessel, but any opening must be immediately covered, 

 for if the flow of blood could not be stemmed by 

 clotting, the slightest cut and many normal functional 

 processes would result in death as a result of un- 

 opposed hemorrhage. 



Normally, the blood contains all the constituents 

 necessary for coagulation, and in addition it contains an 

 antagonistic element, known as ajiti-thrombin, which 

 inhibits intravascular coagulation. A neutralizing 

 agent for anti-thrombin thromboplastin (Howell) 

 is contained in the adjacent tissues and in the outer 

 layers of the walls of the blood vessels themselves. 

 When a blood vessel is cut or torn, thromboplastin 

 comes in contact with and neutralizes antithrombin 

 causing immediate clotting. 



It is obvious that intravascular coagulation would be 

 a more certain method of arresting hemorrhage ; but 

 intravascular coagulation would involve the danger of 

 embolism and the danger of clotting where there is 

 no hemorrhage. To meet cases of extreme danger 



