32 BLOOD. 



other chemical changes being most active at an optimum temperature, and 

 like so many other chemical changes, being assisted by the influences exerted 

 by the presence of inert bodies. 



And we have direct experimental evidence that plasma does contain an 

 antecedent of fibrin which, by chemical change, is converted into fibrin. 



19. If blood be received direct from the bloodvessels 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 moder- 

 ately low temperature, will be deferred for a very long time. If the mixture 

 be allowed to stand, the corpuscles will sink, and a colorless 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 \yheel with several spokes, placed in a hori- 

 zontal position and made to revolve with great velocity (1000 revolutions per min- 

 ute for instance) around its axis. Tubes of metal or of very strong glass are sus- 

 pended at the ends of the spokes by carefully adjusted joints. As the wheel 

 rotates with increasing velocity, each tube gradually assumes a horizontal posi- 

 tion, bottom outward, 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 he 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 colorless 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 removed, 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 sub- 

 stance 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, colorless 

 filtrate, which is at first perfectly fluid. Soon, however, the fluidity gives 

 way to viscidity, and this in turn to a jelly condition, and finally the jelly 

 shrinks into a clot floating in a clear field ; in other words, the filtrate clots 

 like plasma. Thus there is present in cooled plasma, and in plasma kept 

 from clotting by the presence of neutral salts, a something precipitable by 

 saturation with neutral salts a something which, since it is soluble in very 

 dilute saline solutions, cannot be fibrin itself, but which in solution speedily 

 gives rise to the appearance of fibrin. To this substance its discoverer, 

 Denis, gave the name of plasmine. 



The substance thus precipitated is not however a single body, but a mix- 

 turo of at least two bodies. If sodium chloride be carefully added to plasma 

 to an extent of about 13 per cent, a white flaky viscid precipitate is thrown 



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

 salt always clings to the precipitate, and thus the addition of water virtually gives rise 

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



