478 THE BLOOD. 



the clot is smallest under the latter circumstance; and Dr 

 Babington found that the proportion of clot to serum was a.s 

 follows : 



Serum. Clot 



Blood in flask, 10 9 



basin, . .10 17 



Exposure to air favours coagulation, and for this reason 

 whipping or stirring blood leads to rapid clotting. Contact 

 with living tissues prevents coagulation, as well shown by 

 Professor Lister, so that movement within the blood-vessels is 

 adverse to the clotting process. Blood at rest in the body does 

 not consolidate readily unless the tissues are bruised or injured. 

 Tissues seem to retain their vitality for some time after death 

 of an animal, and the blood within them remains fluid. This 

 explains how blood keeps fluid for long in the blood-vessels 

 of dead animals. To show the cause of the blood's fluidity in 

 the body, the late Mr James Turner dissected round the jugular 

 vein of a living horse, placed a ligature around it on the side 

 near the heart, and then one above, so as to include a certain 

 quantity of blood in the tube. On removing this piece of 

 vein from the body, it was found that the blood remained 

 fluid within it for hours, until, by puncturing it, volatiliza- 

 tion occurred and coagulation. In proof of the contact with 

 living tissue having an effect in this process, if the vein is 

 tied and left connected with the tissues in the neck, it becomes 

 congested, inflamed, devitalized in fact, and the blood clots 

 speedily within it. 



From the circumstance that blood clots on being exposed to 

 the air, it was supposed that cold favoured coagulation, but 

 the reverse holds true. Moderate heat favours this process, 

 and cold retards it. Blood may be frozen, and thus trans- 

 formed into a solid mass, which, on thawing, coagulates. 





