no ELEMENTARY PHYSIOLOOY less. 



coagulates when heated to 75" C. (167" F.) as does para- 

 globulin, it differs from paraglobulin and also from 

 fibrinogen by not being precipitated when its solution is 

 saturated with sodium chloride. 



Whilst these three i)roteins are characteristic of the 

 plasma of drawn blood, it would seem that during its 

 circulation in the body there is but one more complex 

 protein from which these are derived. 



The different precipitability of fibrinogen and para- 

 globulin in presence of varying amounts of sodium 

 chloride or other "neutral " salt, such as the sulphates of 

 sodium and magnesium, has been the starting point of all 

 investigations on the processes concerned in the clotting 

 of blood ; we may therefore now proceed very appro- 

 priately to deal with the nature and causes of this 

 striking phenomenon. 



9. The Clotting of Blood. — If a drop of blood be 

 spread out in a thin layer on a slide and kept from 

 drying it soon becomes solid and gelatinous, as in the 

 second experiment described on p. 94. When this solid is 

 carefnlhj washed, by sti'eaming water over it very gently, 

 the colouring matter is removed and a coarse network of 

 extremely delicate fibres or filaments remains. (Fig. 35). 



These filaments are formed in the blood and 

 traversing it in all directions, uniting with one another 

 and binding the corpuscles together, are the cause 

 of the blood having become a semi -solid mass. The 

 filaments are composed of a suV)stance called fibrin ; 

 hence it is this formatioji of fibrin which is the cause of 

 the solidification or clotting of the blood ; but the 

 phenomena of clotting, which are of very great 

 importance, cannot be properly understood until the 

 behaviour of the blood when drawn in much larger 

 quantity than a drop has been studied. 



When, by the ordinary proce.ss of opening a vein with 

 a lancet, a quantity of blood is collected into a basin, 

 it is at first perfectly fluid : but in a very few minutes it 



