478 VETERINARY PHYSIOLOGY 



out of action by the development of some substance which 

 neutralises it. Such a substance is yielded by the breaking 

 down of tissue cells or the cells of the blood, especially the 

 platelets. It is perhaps best called thromboplastin. It is a 

 lipoid compound, probably identical with cephalin. 



The steps in the process might thus be represented as 

 follows : — 



(Fi'om Cells) {In Plasma) 



Prothrombin .H Calcium Ions 



Thromboplastin \ g 



Thrombin'^ ^Fibrinogen 



Fibrin 



Many circumstances influence the rapidity of clotting. Tem- 

 perature has a marked effect, a low temperature retarding 

 it, a slight rise of temperature above the normal of the 

 particular animal accelerating it. If a trace of a neutral 

 salt be added to blood, coagulation is accelerated ; but if 

 blood be mixed with strong solutions of a salt, coagulation 

 is prevented because the formation of thrombin is checked. 

 Calcium salts have a marked and important action, and if 

 they are precipitated by the addition of potassium oxalate, 

 blood will not clot, apparently because thrombin cannot be 

 formed. 



The injection into the blood-vessels of a living animal of 

 commercial peptones, which consist chiefly of proteoses, 

 generally prevents the blood from clotting when shed. This 

 appears to be due to the formation, probably in the liver, of 

 an excess of anti-thrombin. Hirudin, an extract of the 

 head of the medicinal leech, also retards clotting, both when 

 injected into the blood-vessels and when added to the blood 



