PLASMA. 215 



The blood of the horse is chosen because it coagulates more 

 slowly than that of most mammals, and delay in the coagulation 

 or postponement of the change in the plasma is the chief object 

 to be obtained. To encourage this delay the blood is drawn from 

 a vein into a cylinder surrounded with a freezing mixture. The 

 cold, however, must not be so intense as to absolutely freeze the 

 blood, for the wished-for subsidence of corpuscles could not go 

 on if the blood becomes solid. It is then left quite motionless for 

 twenty-four hours, after which time it will be found that the 

 heavy corpuscles have fallen and left a clear supernatant fluid, 

 which is plasma, containing some white cells. This can be re- 

 moved with a cool pipette and passed through an ice-cold filter 

 to remove the cells, then tolerably pure plasma is obtained which 

 soon coagulates at the ordinary temperature. 



Another method of checking coagulation consists of letting the 

 blood flow into a 25 per cent, solution of magnesium sulphate 

 (about three volumes of blood to one of the solution). This, if 

 left in a cool place, will not coagulate, and the corpuscles will 

 separate by subsidence from the plasma and salt solution, which 

 form an upper layer of clear fluid. If the salt be removed by 

 dialysis or weakened by dilution with water, coagulation com- 

 mences. 



The coagulation of plasma can be seen with the microscope to 

 depend upon the appearance of a close feltwork of exquisitely 

 delicate, finely granular elastic fibrils, which pervade the entire 

 fluid, and cause it to set into a soft jelly. The substance forming 

 the meshes is called fibrin. 



Some time after the plasma has gelatinized, the threads of fibrin 

 break away from their attachment to the vessel in which the 

 coagulura is contained, and owing to their elasticity the general 

 mass of fibrin contracts, squeezing out of its meshes clear drops 

 of fluid termed serum. 



The fibrin clot gradually shrinks into unappreciable dimen- 

 sions, and floats in the abundant fluid serum. 



The separation of the serum is accelerated by agitation of the 

 soft clot; and if brisk agitation, such as whipping, be kept up for 

 a few minutes, the plasma does not form a jelly, but the fibrin 



