76 THE BLOOD. 



The coagulation of the blood is retarded by the following 

 means : 



1. Cold retards the coagulation of blood ; and it is said 

 that, so long as blood is kept at a temperature below 40 

 F., it will not coagulate at all. Freezing the blood, of 

 course, prevents its coagulation ; yet it will coagulate, 

 though not firmly, if thawed after being frozen; and it 

 will do so, even after it has been frozen for several months. 

 Coagulation is accelerated, but the subsequent contraction 

 of the clot is hindered, by a temperature between 1 00 and 

 1 20 : a higher temperature retards coagulation, or, by 

 coagulating the albumen of the serum, prevents it 

 altogether. 



2. The addition of water in greater proportion than 

 twice the bulk of the blood. 



3. Contact with living tissues, and especially with the 

 interior of a living blood-vessel, retards coagulation, 

 although if the blood be at rest it does not prevent it. 



4. The addition of the alkaline and earthy salts in the 

 proportion of 2 or 3 per cent, and upwards. When added 

 in large proportion most of these saline substances pre- 

 vent coagulation altogether. Coagulation, however, 

 ensues on dilution with water. The time that blood can 

 be thus preserved in a liquid state and coagulated by the 

 addition of water, is quite indefinite. 



5. Imperfect aeration, as in the blood of those who die 

 by asphyxia. 



6. In inflammatory states of the system, the blood coa- 

 gulates more slowly although more firmly. 



7. Coagulation is retarded by exclusion of the blood 

 from the air, as by pouring oil on the surface, etc. In 

 vacuo, the blood coagulates quickly ; but Prof. Lister thinks 

 that the rapidity of the process is due to the bubbling 

 which ensues from the escape of gas, and to the blood 

 being thus brought more freely into contact with the con- 

 taining vessel. 



