148 PHYSIOLOGY FOR DENTAL STUDENTS. 



completely all the oxalate and leave some in excess. In other 

 words, the presence of a soluble calcium salt is necessary in order 

 to have the blood clot. Defibrinated blood will, however, cause 

 the clotting of pure fibrinogen solutions even though all the cal- 

 cium be removed from both solutions. 



In order to explain the above facts, we must assume that three 



substances are present in solution in the blood: fihrinngfin, cal- 



_cium salts, and another substance, which has been called throrn-^ 



JwQcn. Under the proper conditions, thrombogen will combine 



with calcium salts to form thrombin, which in turn unites with 



fibrinogen to form fibrin, which is the substance forming the 



framework of the clot. 



The reason why the blood does not clot within the blood ves- 

 sels is not definitely known. It is probable that the blood con- 

 tains a substance which prevents the combination of thrombogen 

 with calcium salts, and which we call anti-thrombin. Whenever 

 a blood vessel is injured, the tissues and the blood platelets liber- 

 ate a lipoid body called ' k< plmlin, which unites with the anti 

 thrombin and thus allows the formation of thrombin to take place 

 at the site of the wound. The whole process may be graphically 

 shown in the following schema: 



Anti-thrombin -f- kephalin = inactive anti-thrombin. 

 Thrombogen -(- calcium salts = thrombin. 

 Thrombin -(- fibrinogen = fibrin. 

 Fibrin -f- corpuscles = clot. 



Antibodies in the Blood. The coagulation of the blood is 

 only one of the measures which are developed in the blood for the 

 protection of the animal. No less important in this regard are 

 the destruction and removal of toxic and injurious substances 

 from the body. 



All the infectious diseases are caused by the agency of micro- 

 organisms. The greater number of these are microscopic plants 

 known as bacteria and fungi ; some, however, are unicellular ani- 

 mals known as protozoa. It is especially against the bacteria that 

 a method of defense exists in the body ; the protozoal diseases, on 



