5 THE HUMAy BODY, 



may be removed, and the pure fibrin thus obtained is per- 

 fectly 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 "defi- 

 brinated blood" from which the fibrin has been in this way 

 removed, looks just like ordinary blood, but has lost its 

 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 ob- 

 servation of the process of coagulation; and from the fact 

 that perfectly formed fibrin can be obtained free from cor- 

 'pusclcs 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 the 

 clotting takes place more slowly or the corpuscles sink 

 more rapidly than usual, a colorless top stratum of plasma, 

 with no red corpuscles in it, will be left before gelatiniza- 

 tion occurs and stops the farther sinking of the corpuscles. 

 The uppermost part of the clot formed under these cir- 

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

 "huffy 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 a panacea for all ills. In 

 horse's blood the difference between the specific gravity of 

 the corpuscles and that of the plasma is greater than in hu- 

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

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

 less 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 becomes 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, 



