66 THE BLOOD. 



Conditions affecting Coagulation. 



Although the coagulation of fibrin appears to be spon- 

 taneous, yet it is liable to be modified by the conditions in 

 which it is placed ; such as temperature, motion, the access 

 of air, the substances with which it is in contact, the mode 

 of death, etc. All these conditions need to be considered 

 in the study of the coagulation of the blood. 



The coagulation of the blood is hastened by the follow- 

 ing means : 



1. Moderate warmth, from about 100 F. to 120 K 



2. Rest is favourable to the coagulation of blood. Blood, 

 of which the whole mass is kept in uniform motion, as 

 when a closed vessel completely filled with it is constantly 

 moved, coagulates very slowly and imperfectly. But rest 

 is not essential to coagulation ; for the coagulated fibrin 

 may be quickly obtained from blood by stirring it with 

 a bundle of small twigs ; and whenever any rough points 

 of earthy matter or foreign bodies are introduced into 

 the blood-vessels, the blood soon coagulates upon them. 



3. Contact with foreign matter, and especially multi- 

 plication of the points of contact. Thus, when all other 

 conditions are unfavourable, the blood will coagulate upon 

 rough bodies projecting into the vessels ; as, for example, 

 upon threads passed through arteries or aneurismal sacs, 

 or the heart's valves roughened by inflammatory deposits 

 or calcareous accumulations. And, perhaps, this may 

 explain the quicker coagulation of blood after death in the 

 heart with walls made irregular by the fleshy columns, 

 than in the simple smooth-walled arteries and veins. 



4. The free access of air. 



5. Coagulation is quicker in shallow, than in tall and 

 narrow vessels. 



6. The addition of less than twice the bulk of water. 

 The blood last drawn is said to coagulate more quickly 



than that which is first let out. 



