102 A TEXTBOOK OF PHYSIOLOGY 



(vii.) By injecting into an animal before killing it certain substances 

 such as commercial peptone, soap solution, or, very slowly, a weak 

 alkaline solution of nucleoprotein. 



(viii.) The addition of acids, alkalies, ammonia sugar solution, 

 glycerine, or much water. 



Clotting may be facilitated, on the other hand 



(i.) By keeping the temperature that of the body. 



(ii.) By injuring the wall of the containing bloodvessel. 



(iii.) By receiving on to a rough surface to which the blood adheres; 

 by beating it with twigs or shaking it with glass beads. The addition 

 of finely powdered carbon or platinum black also quickens coagulation. 



(iv.) By adding serum or blood-clot. 



(v.) By adding saline extract of lymphatic glands and other tissues. 



(vi.) Possibly by the addition of soluble calcium salts. 



The explanation of all the above facts in regard to the clotting of 

 blood is a matter of great difficulty. Opinions strongly at variance 

 are held in regard to the exact processes which take place. 



The following seem to be the certain facts about the clotting of 

 blood, whatever may be the interpretation of the same : 



(i.) When blood clots the protein of the plasma known as fibrinogen 

 is involved and becomes converted either partially or wholly into a 

 solid body known as fibrin. This is shown by the experiment that 

 fibrinogen may be precipitated from plasma, redissolved in saline, 

 and clotted at 37 C., by the addition of a trace of blood-serum or a 

 watery extract of serum proteins coagulated by alcohol. 



(ii.) Calcium ions are necessary for the process. Thus, the addition 

 to blood, as it is shed, of a soluble oxalate or fluoride which precipitates 

 the calcium ions, or of a soluble citrate which prevents their dissocia- 

 tion, stops the coagulation of the blood. 



(iii.) Calcium ions take part in an intermediate and not in the 

 final process, since a calcium-free solution of fibrinogen may be clotted 

 by the addition of calcium-free blood-serum i.e., blood which has 

 already clotted. 



(iv.) Tissue juice has the property of greatly accelerating the 

 process of clotting. Bird's blood straight from the vessel does not 

 clot; if tissue extract be added, the blood clots almost at once. The 

 addition of lymph from a blister accelerates the clotting of human 

 blood. 



(v.) Adhesion between the blood and a foreign substance gives 

 an impulse towards coagulation, while lack of such adhesion prevents 

 the blood from clotting. 



The explanation given of the above facts is that blood, flowing from 

 a wound, becomes mixed with the tissue fluids in the cut, and the 

 blood with the tissue fluid in the presence of calcium ions forms an 

 enzyme known as thrombin from a forerunner present in the blood, 

 known as thrombogen or prothrombin, which is probably derived 

 from the white corpuscles and blood-platelets. It is only when tissue 

 juices and calcium ions are present that this enzyme formation takes 

 place. This explains why a jagged wound clots more readily than 



