96 APPLIED PHYSIOLOGY 



constant as the composition of the blood itself, variations 

 depending only on variations in the capacity of the 

 vascular system. In chlorosis the total volume of blood 

 is increased, and the constant tension of the vessels is 

 believed to be the cause of the vascular murmurs in that 

 disease. In no form of anaemia is the total volume of 

 the blood constantly diminished.* 



Clotting. The power of the blood to clot is one of 

 its most convenient attributes. Were it not for this, we 

 should all be liable to bleed to death from any trivial 

 wound of a bloodvessel. 



Much ingenious experiment has been expended in 

 attempts to explain the true inwardness of clotting and 

 what really happens when it takes place, but it cannot 

 be said that we are even yet completely informed on all 

 details. It is generally agreed, however, that clotting 

 is due to the conversion of the soluble globulin called 

 fibrinogen into insoluble fibrin, and that this change 

 takes place under the influence of a ferment called 

 thrombin, which, however, is not present in the living 

 blood as such, but is liberated by the breaking down of 

 white corpuscles, and possibly of platelets. Thrombin 

 belongs, apparently, to the nucleo- protein class, and 

 exists as a * zymogen ' (prothrombin) which only be- 

 comes active in the presence of soluble salts of lime. 



It is difficult to say what the rate of clotting of blood 

 outside the body is, as it varies greatly according to the 

 method employed to determine it. It seems certain, 

 however, that the rate is very variable, not only in 



* See Lorrain Smith, Trans, of the Path. Soc. of London, 1900, 

 li. part 311. 



