222 MANUAL OF PHYSIOLOGY. 



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 commences. 



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 form- 

 ing 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 coagulum 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 

 firmly adheres to the stirring rods and at once contracts around 

 them. 



