THE CLOTTING OF BLOOD. 57 



be collected over mercury from a blood-vessel, without having 

 been exposed to the air even for an instant, it will clot per- 

 fectly. 



The formation of fibrin is then due to changes taking 

 place in the blood itself when it is removed from the blood- 

 vessels; the clotting depends solely upon some rearrangement 

 of the blood-constituents, and the primary change seems to 

 be the formation of fibrin-ferment. That healthy circulating 

 blood contains no ferment but that this forms in drawn blood 

 may be shown as follows: Blood is drawn from an artery 

 into four separate vessels. To one specimen a large quantity 

 of alcohol is added at once; to a second after five minutes, to 

 a third after ten, to the fourth after fifteen. The precipitate 

 in each is collected and dried, and then treated with water 

 which will dissolve any ferment present. The watery extract 

 from the first specimen will not cause clotting when added to 

 a fibrinogen solution: from the second only slowly; the third 

 more quickly, and the fourth quickest of all. It is hence con- 

 cluded that there is no ferment in perfectly fresh blood, but 

 that this begins to form as soon as blood is drawn and for 

 some time goes on increasing, so that there is more in blood 

 drawn ten minutes than in blood drawn only five. The 

 alcohol in each sample precipitates all the ferment already 

 present and prevents the formation of more. There is some 

 evidence that a good many pale corpuscles disintegrate when 

 blood is drawn, and it has been maintained that they then 

 give origin to the fibrin-ferment along with other things : but 

 of late evidence seems rather to point to the platelets as 

 the main source of the ferment. As already stated they 

 rapidly break down when blood is removed from the body, 

 part of their substance going into solution in the plasma and 

 part remaining as a sticky mass which tends to adhere to its 

 fellows to form little clumps. If the formation of fibrin in 

 clotting blood be watched with the aid of a microscope the 

 fibrin threads are seen to appear first in the neighborhood of 

 these clumps, and in many cases to radiate from them. More- 

 over those substances which check or retard the clotting of 

 blood also hinder the disintegration of the platelets: 'and if a 

 fine thread be passed through the blood-vessel of a living 

 animal fibrin forms around it after a time, and this formation 

 is preceded by adhesion to the thread and disintegration of 

 platelets. But be the source of the ferment platelets or pale 



