170 BLOOD AJ^TD LYMPH 



out of the serum or defibrinated plasma. Wtat the cause is that 

 suddenly produces this coagulation of fibrinogen, and the conse- 

 quent formation of fibrin fibers, the experiment failed to explain. 

 While this question has not yet been fully answered the theory 

 which is generally accepted at the present time may be stated as 

 follows: 



The experimental facts upon which the theory is based are 

 three. First, the coagulation of fibrin is not due to exposure to 

 air, as it takes place with equal readiness when air is not present. 

 Second, it occurs only when there are present calcium salts, fibrin- 

 ogen, and an enzyme called thrombin. Third, the enzyme throm- 

 bin is not a part of the normal composition of plasma, but is 

 furnished under special conditions by the combined action of 

 the blood plates, the leucocytes, and the calcium salts. 



The formation of the clot involves the following actions: First, 

 the breaking down of the blood plates and leucocytes in the 

 presence of calcium salts results in the formation of a chemical 

 compound or enzyme called thrombin. The enzyme reaction of 

 this thrombin causes the coagulation of fibrinogen into fibrin. 

 The reason that this action occurs only when the blood is removed 

 from the blood vessels is probably due to the fact that the blood 

 plates and leucocytes do not disintegrate in sufficient numbers 

 while the blood is in the body to produce much thrombin. It is 

 also possible that in blood which is inclosed in blood vessels there 

 may be compounds among the unknown constituents of plasma 

 which neutrahze the thrombin as fast as it is formed. 



Conditions affecting the blood. — Since the blood fur- 

 nishes the oxygen and food supply of the tissues, it is 

 evident that it must be maintained in such a condition 

 as to perform these functioas. Anything, therefore, which 

 interferes with the number of corpuscles, the presence 

 of hiTmoglobin, the oxygen content of the corpuscles, or 

 tlie food content of the plasma, is bound to interfere 

 with its normal functions. The conditions which have 

 been found to be essential to a healthy condition of blood 



