THE BLOOD. 71 



to form material amounts of fibrin ferment, and so the 

 blood is prevented from clotting. Though this explanation 

 at first sight seems sufficient, a closer study shows that it 

 does not yet answer all the observed facts in coagulation of 

 blood, and there are many observers who offer modified 

 theories to account for this phenomenon. Thus, we do not 

 know from what source the fibrinogen is derived. Al. 

 Schmidt, the originator of the theory here given, believes 

 that the fibrinogen and the fibrinoplastin both, as well as 

 fibrin ferment, are derived from the disintegration of white 

 corpuscles. This might explain why at the point where 

 clotting begins and where, therefore, large numbers of 

 these corpuscles are dissolved, added amounts of fibrinogen 

 should form and thus facilitate clotting. 



But to recapitulate : The best explanation now available 

 is that there is present in normal human blood dissolved in 

 the plasma about two-tenths per cent, of an albumen known 

 as fibrinogen. This fibrinogen in the presence of common 

 salt and lime salts will change into insoluble fibrin threads 

 under the influence of this fibrin ferment, at present believed 

 by all to be derived from the colorless corpuscles of the 

 blood. The property of coagulation is by no means pecu- 

 liar to fibrinogen. Most albumens possess it more or less 

 fully shown. The liquid casein of milk easily coagulates 

 into cheese under the action of the rennet ferment. The 

 liquid albumen of living muscles soon clots after death, 

 producing the so-called death stiffening, or rigor-mortis. 

 Even ordinary egg albumen can easily be made to coagu- 

 late by the action of heat or chemical agents. 



The liquid that is left after the blood has clotted is 

 called serum. 



THE COMPOSITION OF SERUM. 



The serum of ordinary blood contains about ninety per 

 cent, of water. It contains two albumens which have indi- 

 rectly taken part in the formation of the clot called fibrino- 

 plastin and serum albumen. This serum albumen must not 



