112 ELExVlEXTARY PHYSIOLOGY less. 



but in this case the clot is broken up as fast as it tends to 

 be formed. Under these circumstances the fibrin collects 

 upon the twigs, and a red fluid is left behind, consisting 

 of the serum j)lus the red corpuscles and many of the 

 colourless ones. The fibrin adhering to the twigs may 

 readily be washed in a stream of water, and as thus 

 obtained is a white, stringy, elastic and very insoluble 

 substance. It gives, when tested, all the reactions 

 characteristic of proteins, and is in fact itself a protein, 

 although somewhat impure. 



The clotting of the blood is hastened, retarded, or 

 temporarily prevented by many circumstances. 



(a) Temperature. — A temperature up to or slightly 

 above 40' C. (104' F.) accelerates the clotting of the 

 blood ; a low one retards it very greatly ; so much so 

 that blood kept at a temperature close to freezing 

 point may remain fluid for a very long time indeed. 



(b) The addition of neutral salts to the blood. — Many 

 salts, .and more esjiecially sulphate of sodium or mag- 

 nesium and sodium chloride (connnon salt), dissolved in 

 the blood in sufficient quantity, prevent its clotting ; 

 but clotting sets in when water is added, so as to 

 dilute the saline mixture. 



(c) Contact ivith living or not living matter. — Contao# 

 with not living matter promotes the clotting of the 

 blood. Thus, blood drawn into a basin begins to clot 

 first where it is in contact with the sides of the 

 basin ; and a wire introduced into a living vein will 

 become coated with fibrin, although perfectly fluid blood 

 surrounds it. 



In its normal surroundings, for instance in a portion of 

 a vein which is tied at each end, blood remains fluid for a 

 very long time. The lieart of a turtle remains alive for a 

 lengthened period (many hours or even days) after it is 

 extracted from the body ; and, so long as it remains 

 alive, the blood contained in it will not clot, though, 

 if a portion of the same blood be removed from the 



