120 STUDIES IN PHYSIOLOGY 



the slaughterhouse to catch a quart of freshly drawn blood, 

 and to stir it vigorously for several minutes with a bunch of 

 twigs or a broom. On examining the twigs one sees that 

 they are covered with a stringy mass. When this is washed, 

 it is found to be composed of white elastic fibers, like those 

 we saw forming in the drop of blood under the compound 

 microscope. By testing this so-called blood fibrin with nitric 

 acid and ammonia, we demonstrate it to be a kind of proteid. 



Defibrinated Blood. After the fibrin has been removed, a 

 red liquid remains that looks like the normal blood. But 

 however long it is allowed to stand, it will never clot. The 

 name de-fi'bri-na-ted blood (Latin de = without +fibriri) is 

 given to this liquid. So then we have proved in two differ- 

 ent ways that the clotting of blood is due to blood fibrin. 



Difference between Blood Plasma and Blood Serum. We 

 have called the freshly drawn liquid in which the blood cor- 

 puscles float, the blood plasma, and the straw-colored liquid 

 surrounding the clot, blood serum. While the appearance 

 of both liquids under the compound microscope is much the 

 same, they differ in one particular : blood plasma clots, blood 

 serum does not. Or to describe their difference in composi- 

 tion, we may say that blood serum is blood plasma minus 

 fibrin. 



The question naturally presents itself, Why does not 

 blood plasma clot when it is in the body? Physiologists 

 have demonstrated that coagulation is not due to the fact 

 that the blood has ceased to be in motion, nor is it caused by 

 exposure to the air. It is known that a liquid proteid called 

 fi-brin'o-gen (Latin fibrin-}- gen = maker) is found in blood 

 plasma, and that this is changed into solid fibrin when a 

 clot is formed. But why this change does not take place as 

 long as the blood is within a healthy blood vessel, has not 

 been satisfactorily explained. 



Composition of Blood Serum. Blood serum contains over 

 90 % of water, in which are dissolved the various nutrients 

 obtained by absorption from the alimentary canal. The 



