BLOOD AND LYMPH 205 



Many circumstances influence the rapidity of clotting. 



Temperature 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 salt, coagulation is prevented 

 because the formation of thrombin is checked. Calcium salts 

 have a marked and important action, and if they are precipi- 

 tated by the addition of oxalate of soda, blood will not clot, 

 apparently because thrombin cannot be formed. 



The injection into the blood vessels of a living animal of 

 commercial peptones, which chiefly consist of proteoses, or of 

 hirudin, an extract of the head of the medicinal leech, retards 

 coagulation after the blood is shed. They appear to cause the 

 development in the liver of some body of the nature of an anti- 

 thrombin which checks coagulation, and if the liver be excluded 

 from the circulation this is not developed 



Why is it that blood does not coagulate in the vessels and 

 does coagulate when shed ? Such a general statement is not 

 absolutely correct, for blood may be made to coagulate in the 

 vessels of a living animal in various ways. If inflammation is 

 induced in the course of a vessel, coagulation at once occurs. 

 If the inner coat of a vessel be torn, as by a ligature, or if any 

 roughness occurs on the inner wall of a vessel, coagulation is 

 apt to be set up. Again, various substances injected into the 

 blood stream may cause the blood to coagulate, and thus 

 rapidly kill the animal. Among such substances are extracts 

 of various organs thymus, testis, and lymph glands which 

 yield thrombokinase, and snake venom which seems to 

 contain active thrombin. The injection of pure thrombin 

 does not usually cause clotting, because an anti-thrombin is 

 developed. 



Nor does blood necessarily coagulate when shed. If it is re- 

 ceived into castor oil, or into a vessel anointed with vaseline 

 and filled with paraffin oil, it will remain fluid for a consider- 

 able time. Apparently some roughness in the wall of the blood 

 vessel or of the vessel in which the blood is received is required 

 to start the process, acting as a focus from which it can spread 

 outwards. 



The advantages of coagulation of blood are manifest. By 



