MILK, BLOOD AND URINE 225 



the blood, ^'.e. corpuscles, etc., which are held mechanically and 

 carried down with the fibrin. 



If fresh-drawn blood is beaten rapidly with a bundle of fine 

 twigs or of strings, the coagulating fibrin will be caught on the 

 twigs, while most of the corpuscles remain in the serum. Blood 

 so treated is called de-fibrinated. The fibrin so separated may 

 be used for testing the color reactions of proteins or the diges- 

 tible action of proteolytic enzymes. It is a white fibrous 

 material and responds to all protein tests. 



Under normal conditions, freshly drawn blood will clot in 

 from two to ten minutes, though the rapidity of clotting varies 

 a good deal in different animals and is especially slow under 

 certain pathological conditions as in fever. The separation of 

 the clot from the serum takes place very slowly, requiring from 

 ten to forty-eight hours. The time taken for clotting may be 

 shortened very greatly by certain treatment and is often done 

 in controlling hemorrhages. A solution of ferric chloride or 

 alum applied to a fresh cut produces an almost instantaneous 

 clotting. The natural clotting may be hastened or retarded 

 by the injection of certain substances, e.g. calcium salts. In 

 general an increase of carbon dioxide and decrease of oxygen 

 causes blood to clot more slowly and vice versa. Snake venom 

 and toxic albumin cause marked retardation in clotting ; and if 

 blood is drawn into a solution of ammonium oxalate, it will not 

 clot. 



The reason why blood does not clot until exposed to the air 

 is not wholly explained, but is considered to be due to the 

 presence in the normal blood of an anti-enzyme called anti- 

 thrombin which prevents the action of thrombin upon fibrino- 

 gen. It is also probably true that very little if any thrombin 

 is present in the blood within the blood vessels, as the leuco- 

 cytes do not yield prothrombin until the action of air causes 

 them to disintegrate. 



