THE BLOOD 117 



blood is shed there is at once the death of 

 many colourless cells. These contain a pro- 

 tein called pro-thrombin, which, in turn, 

 produces an enzyme known as thrombin, and 

 this ferment, in association with salts of lime, 

 converts fibrinogen into fibrin. To account 

 for the fact that blood rarely clots in living 

 vessels, we may assume the existence of a 

 body produced by the living cells lining the 

 vessels which prevents thrombin from acting 

 (an anti- thrombin). There is still uncertainty 

 as to what precisely happens in the remark- 

 able phenomenon of the clotting of blood, 

 and there is little doubt that if it were 

 thoroughly understood, light would be thrown 

 on other physiological phenomena, as it may 

 be taken as the type of a certain class of 

 changes in living matter. 



60. The blood is not only a nutritional 

 medium but it is also intimately connected 

 with respiration. The red cells, by the action 

 of the pigment they contain, known as 

 haemoglobin, are engaged in carrying oxygen 

 to the tissues and also it would appear to 

 some extent in the carrying of carbonic acid 



