The fluid part of the blood, plasma or liquor sanguinis, is of a pale straw color, when pure and free from the coloring matter of the corpuscles, and of slightly less density (p. 215).

Unless special precautions have been taken, the plasma is altered when removed from the blood vessels and coagulation of the blood takes place, so that plasma does not come under observation, except when suitable methods are employed to separate it from the corpuscles. It was first separated from the corpuscles by the filtration of frog's blood to which strong syrup had been added to delay coagulation and destroy the flexibility of the corpuscles, so that they were rapidly caught in the meshes of the filter and the clear plasma passed through.

Reticulum of Fibrin Threads after staining has made them visible.

Fig. 96. Reticulum of Fibrin Threads after staining has made them visible. The network (b) appears to start from granular centres (a). (Kanvier).

To obtain mammalian plasma free from corpuscles it is necessary to use some other method, as the small elastic corpuscles easily run through the meshes of the thickest filter paper.

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 became 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 removed 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 in 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 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 to small size 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 in recently drawn blood, the plasma does not form a jelly, but the fibrin firmly adheres to the stirring rods and at once contracts around them.