ELEMENTARY PHYSIOLOGICAL CHEMISTRY 191 



ferment (see ferments, p. 214, and for preparation see p. 476). On 

 the other hand, the addition of a soluble oxalate must have 

 removed from the plasma some necessary agency, namely, soluble 

 calcium salts. It does this by forming calcium oxalate, which is in- 

 soluble. We can prove that this explanation is the correct one by the 

 following experiments : 



EXPERIMENT II. To the test-tube c in the above experiment add 

 about 5 drops of a 2 per cent, solution of calcium chloride ; then replace 

 in water-bath. Clotting now occurs, since soluble calcium salts are 

 present in excess. 



EXPERIMENT III. Clotting has been prevented in blood by mixing 

 it with one-quarter its volume of a 1 per cent, solution of potassium 

 oxalate in normal saline. Add to a few c.c. of this oxalate blood in a 

 test-tube a few drops of a 2 per cent, solution of CaCl 2 , and place in 

 water-bath. Clotting occurs. 



We see, then, that the plasma must contain some proteid in solution 

 which is rendered insoluble by the action of a ferment acting in the 

 presence of soluble calcium salts. This soluble proteid can be precipi- 

 tated from the plasma by half saturating with sodium chloride, and it 

 is called Fibrinogen (for preparation see p. 476). 



Since no fibrin ferment can be prepared from absolutely fresh blood, 

 it must exist there as a precursor, and since the greatest yield of it is 

 obtained from that portion of the clot containing most leucocytes, 1 

 it must be in these that this precursor exists. The exact chemical nature 

 of this substance is unknown, but there is much evidence to show that 

 it is of the nature of nucleo-proteid. Fibrin ferment is sometimes 

 called thrombin, and the precursor pro-thrombin, and it is supposed that, 

 when the latter changes into the former, calcium salts are added to it. 



To sum up, therefore, the process of dotting begins by the leucocytes 

 disintegrating and liberating pro-thrombin, which immediately combines with 

 soluble calcium salts to form thrombin. This thrombin then acts on fibrinogen 

 and splits it into two bodies, the one, unimportant, remains in solution 

 (Hammarsten's secondary globulin), and the other solidifies and forms fibrin. 2 

 This fibrin is thrown down as fine threads which cross one another in 

 all directions, forming a meshwork in which the corpuscles become 

 entangled when the process occurs in unseparated blood. If fresh 

 .blood be whipped with a bunch of twigs, the strings of fibrin collect 

 on these, and the fibrin can thus be removed from the serum and 



1 I.e. from the huffy coat, which is the paler upper portion^ of the clot of bloods, 

 such as horses' blood, which clot slowly. 



2 It has been supposed that the thrombin transfers its calcium salts to this 

 fraction of the fibrinogen molecule, and that fibrin is really Jibrinate of calcium. 



