314 THE HUMAN BODY 



material will be found attached to them. This is at first colored 

 red by adhering blood-corpuscles: but by washing in water they 

 may be removed, and the pure fibrin thus obtained is perfectly 

 white and in the form of highly elastic threads. It is insoluble in 

 water and in dilute acids, but swells up to a transparent jelly in 

 the latter. The "whipped" or " defibrinated blood" from which 

 the fibrin has been in this way removed, looks just like ordinary 

 blood, but has lost the power of coagulating spontaneously. 



The Buffy Coat. That the red corpuscles are not an essential 

 part of the clot, but are merely mechanically caught up in it, 

 seems clear from the microscopic observation of the process of 

 coagulation; and from the fact that perfectly formed fibrin can 

 be obtained free from corpuscles by whipping the blood and 

 washing the threads which adhere to the twigs. Under certain 

 conditions, moreover, one gets a naturally 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 cor- 

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

 cumstances is, colorless or pale yellow, and is known as the buffy 

 coatj, 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- 



