THE BLOOD. 67 



THE COAGULATION OF THE BLOOD. 



It is a matter of common observation that when the blood 

 is allowed to stream from a severed vessel into a jar it forms 

 in a very short time a solid, trembling jelly. This happens 

 in the blood of a pigeon almost instantaneously; in the 

 blood of a horse, coagulation is slower, while in man it is 

 medium. Soon the top of the jelly or clot becomes cupped, 

 and a transparent or slightly colored liquid appears over it. 

 This cupping is due to the contraction of the clot, which, 

 continuing, soon pulls the clot loose from the walls of the 

 vessel, and by its continued contraction forces out larger 

 and larger quantities of the liquid just referred to, which is 

 called serum. If the blood had been prevented from clot- 

 ting so rapidly, which might have been done by subjecting 

 it to a cold temperature, the corpuscles would have settled 

 to the bottom, leaving a clear liquid on top. As the white 

 corpuscles are not quite as heavy as the red, they are the 

 last to settle, and there is formed a whitish layer over the 

 top called the "buffy coat." 



As soon as the temperature is raised the blood begins to 

 clot. If blood, however, as it is streaming into the jar be 

 stirred with a stick, or as we say, "whipped," it does not 

 clot regularly at all ; but there may be seen collected on the 

 stick a bundle of threads whose removal from the clot has 

 prevented the "blood from solidifying. If these threads be 

 removed from the stick and carefully washed to free them 

 from entangled corpuscles, they are seen to consist of a 

 mass of stringy matter which has been called fibrin. Evi- 

 dently the clotting of the blood is due to the fonnati6n of 

 these threads all through the clot, in the meshes of which 

 threads the corpuscles are mechanically included. The 

 later contraction of these threads squeezes out the con- 

 tained serum. The difference, therefore, between blood 

 plasma and blood serum is the absence of the fibrin from 

 the latter. Just from what this fibrin comes, and what are 

 the causes that have led to its sudden formation are ques- 

 tions for further study. 



