FUNCTIONS OF THE BLOOD 281 



gelatinization occurs and stops the further sinking of the cor- 

 puscles. The uppermost part of the clot formed under such cir- 

 cumstances 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 blood-letting 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, however, not be confounded with another phenomenon. 

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

 comes bright scarlet, while the part immersed in the serum has 

 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 com- 

 pound with the coloring matter of the red corpuscles. If the 

 clot be turned upside down and left for a short time, the pre- 

 viously dark red bottom layer, now exposed to the air, becomes 

 bright. 



Uses of Coagulation. The clotting of the blood is so important 

 a process that its cause has been frequently investigated ; 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 bleeding, which would otherwise go on in- 

 definitely, is stopped. So, too, when a surgeon ties up an artery 

 before dividing it, the tight ligature crushes or tears its delicate 

 inner surface, and the blood clots where that is injured, and from 

 there a coagulum is formed reaching up to the next highest branch 

 of the vessel. This becomes more and more solid, and by the time 

 the ligature is removed has formed a firm plug in the cut end of 

 the artery, which greatly diminishes the risk of bleeding. 



The Source of Blood-Fibrin. Since fresh blood-plasma contains 

 no fibrin but does contain considerable quantities of other pro- 

 teins, we look first to these as a possible source of the fibrin formed 

 during coagulation. If horse's blood be drawn directly from the 



