THE CLOTTING OF BLOOD. 53 



formed fibrin can be obtained free from corpuscles by whip- 

 ping the blood and washing the threads which adhere to the 

 twigs. Under certain conditions, moreover, one gets a natu- 

 rally formed clot containing no red corpuscles in one part of 

 it. The corpuscles of human blood are a little heavier, bulk 

 for bulk, than the plasma in which they float; hence, when 

 the blood Is drawn and left at rest they sink slowly in it; 

 and if for any reason clotting take place more slowly or the 

 corpuscles sink more rapidly than usual, a colorless top 

 stratum of plasma, with no red corpuscles in it, is left 

 before gelatinization occurs and stops the further sinking of 

 the corpuscles. The uppermost part of the clot formed 

 under such circumstances is colorless or pale yellow, and is 

 known as the buffy coat; it is especially apt to be formed in 

 the blood drawn from febrile patients, and was therefore a 

 point to which physicians paid much attention in the olden 

 times when bloodletting was thought to be almost a panacea. 

 In horse's blood the difference between the specific gravity of 

 the corpuscles and that of the plasma is greater than in 

 human blood, and horse's blood also coagulates more slowly, 

 so that its clot has nearly always a buffy coat. The colorless 

 buffy coat seen sometimes on the top of the clot must, how- 

 ever, not be confounded with another phenomenon. When 

 a blood-clot is left floating exposed to the air its top becomea 

 bright scarlet, while the part immersed in the serum assumes 

 a dark purple-red color. The brightness of the top layer is 

 due to the action of the oxygen of the air, which forms a 

 scarlet compound with the coloring matter of the red cor- 

 puscles. If the clot be turned upside down and left for a 

 short time, the previously dark red bottom layer, now exposed 

 to the air, becomes bright; and the previously bright top 

 layer, now immersed in the serum, loses iis oxygen and be- 

 comes dark. 



Uses of Coagulation. The clotting of the blood is so im- 

 portant a process that its cause has been frequently investi- 

 gated; but it is not yet completely understood. The living 

 circulating blood in the healthy blood-vessels does not clot; 

 it contains no solid fibrin, but this forms in it, sooner or later, 

 when the blood gets by any means out of the vessels or when 

 the lining of these is injured. In this way the mouths of the 

 small vessels opened in a cut are clogged up, and the bleed- 

 ing, which would otherwise go on indefinitely, is stopped. 



