22 PLASMA. [BOOK i. 



19. If blood be received direct from the blood-vessels into 

 one-third its bulk of a saturated solution of some neutral salt such 

 as magnesium sulphate, and the two gently but thoroughly mixed, 

 clotting, especially at a moderately low temperature, will be 

 deferred for a very long time. If the mixture be allowed to stand, 

 the corpuscles will sink, and a colourless plasma will be obtained 

 similar to the plasma gained from horse's blood by cold, except 

 that it contains an excess of the neutral salt. The presence of 

 the neutral salt has acted in the same direction as cold: it has 

 prevented the occurrence of clotting. It has not destroyed the 

 fibrin ; for if some of the plasma be diluted with from five to ten 

 times its bulk of water, it will clot speedily in quite a normal 

 fashion, with the production of quite normal fibrin. 



The separation of the fluid plasma from the corpuscles and from 

 other bodies heavier than the plasma is much facilitated by the use of 

 the centrifugal machine. This consists essentially of a tireless wheel 

 with several spokes, placed in a horizontal position and made to revolve 

 with great velocity (1000 revolutions per minute for instance) round 

 its axis. Tubes of metal or very strong glass are suspended at the 

 ends of the spokes by carefully adjusted joints. As the wheel rotates 

 with increasing velocity, each tube gradually assumes a horizontal 

 position, bottom outwards, without spilling any of its contents. As 

 the rapid rotation continues the corpuscles and heavier particles are 

 driven to the bottom of the tube, and if a very rapid .movement be 

 continued for a long time will form a compact cake at the bottom 

 of the tube.- When the rotation, is stopped the tubes gradually return 

 to their upright position again without anything being spilt, and the 

 clear plasma in each tube can then be decanted off. 



If some of the colourless transparent plasma, obtained either 

 by the action of neutral salts from any blood, or by the help of cold 

 from horse's blood, be treated with some solid neutral salt, such as 

 sodium chloride, to saturation, a white flaky, somewhat sticky 

 precipitate will make its appearance. If this precipitate be re- 

 moved, the fluid no longer possesses the power of clotting (or very 

 slightly so), even though the neutral salt present be removed by 

 dialysis, or its influence lessened by dilution. With the removal 

 of the substance precipitated, the plasma has lost its power of 

 clotting. 



If the precipitate itself, after being washed with a saturated 

 solution of the neutral salt (in which it is insoluble) so as to get 

 rid of all serum and other constituents of the plasma, be treated 

 with a small quantity of water, it readily dissolves 1 , and the solution 

 rapidly filtered gives a clear colourless filtrate, which is _at_ first 

 perfectly fluid. Soon however the fluidity gives way to viscidity, 



1 The substance itself is not soluble in distilled water, but a quantity of the 

 neutral salts always clings to the precipitate, and thus the addition of water virtually 

 gives rise to a dilute saline solution, in which the substance is readily soluble. 



