Ill THE CLOTTTNCx OF BLOOD 113 



heart, it will clot in a few minutes. Blood taken from 

 the body of the turtle, and kept from clotting by cold 

 for some time, may be poured into the separated, but 

 still living, heart, and then will not clot. 



Even more remarkable is the immunity from clotting 

 which is conferred upon blood by the salivary secretion 

 of the leech. Blood which has been contauiinated with 

 even minute traces of this substance will remain fluid 

 indefinitely in an open vessel. 



The clotting of blood being thus due to the appearance 

 in it of fibrin we may now consider how and why it is 

 formed when blood is .shed 



The exact natui-e of the changes involved in the 

 formation of fibrin have not even yet been thoroughly 

 worked out ; but the following facts throw a good deal of 

 light on them : — The pericardium and other serous cavities 

 in the body, contain a clear fluid, which may be briefly 

 described as consisting of the elements of the blood 

 without the red blood-corpuscles. This fluid sometimes 

 clots spontaneously, as the blood plasma would do, but 

 very often shows no disposition to clot spontaneously, 

 especially if it is not removed from the body until some 

 hours after death. When the latter is the case, the fluid 

 may nevertheless be made to clot and yield true fibrin, by 

 adding to it a few drops of whipped blood, i.e. of blood 

 which has clotted, or a little serum of blood. Now if a 

 specimen of pericardial fluid, which has been thus observed 

 not to clot spontaneously, but to clot readily on the 

 addition of blood or serum, be treated with salt in the 

 same way as described above for blood plasma, a substance 

 will be thrown down which, at first siglft, looks exactly 

 like that thrown down from blood plasma under similar 

 circumstances. But there is a great diflference, for the 

 substance thus obtained from pericardial fluid when 

 dissolved in water will not clot spontaneously, though its 

 solutions may be made to clot at any time by the addition 

 of a little serum, or whipped blood. It is clearly an 



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