464 BLOOD AND LYMPH. 



or be prevented altogether by a variety of means, of which the 

 following are the most important: 



1. By Cooling. — This method succeeds well only in blood 

 that clots slowly — for example, the blood of the horse, bird, or 

 terrapin. Blood from these animals received into narrow vessels 

 surrounded by crushed ice may be kept fluid for an indefinite 

 time. The blood corpuscles soon sink, so that by this means 

 one may readily obtain pure blood-plasma. The cooling probably 

 prevents clotting by keeping the corpuscles intact. 



2. By the Use of Oiled or Paraffined Vessels. — If blood, when 

 withdrawn from the vessels, is allowed to come into contact only 

 with oiled or paraffined surfaces its clotting is very much delayed. 

 Under ordinary conditions blood brought into contact with 

 foreign surfaces clots rapidly, because the platelets quickly agglu- 

 tinate and dissolve, yielding to the plasma a supply of prothrom- 

 bin and thromboplastic substance. An oiled or paraffined surface 

 is one which the liquid of the blood cannot wet, and under these 

 conditions the platelets do not undergo the rapid change in 

 surface properties that causes them to break down. The cannula 

 used for drawing the blood and the vessel for receiving it may be 

 paraffined conveniently by filling with a dilute solution of paraffin 

 in ether, then pouring off the solution and allowing to dry for a 

 few minutes. In this way the glass surface is coated with a thin, 

 uniform layer of paraffin. 



3. By the Action of Neutral Salts. — Blood received at once 

 from the blood-vessels into a solution of such neutral salts as 

 sodium sulphate or magnesium sulphate, and well mixed, does 

 not clot. In this case also the corpuscles settle slowly, or they 

 may be centrifugalized, and specimens of plasma be obtained. 

 For this purpose horses' or cats' blood is to be preferred. Such 

 plasma is known as "salted plasma"; it is frequently used in 

 experiments in coagulation — for example, in testing the efficacy 

 of a given thrombin solution. The best salt to use is magnesium 

 sulphate in solutions of 27 per cent.: 1 part by volume of this 

 solution is usually mixed with 4 parts of blood; if cats' blood is 

 used, a smaller amount may be taken — -1 part of the solution to 

 9 of blood. Salted plasma or salted blood again clots when 

 diluted sufficiently with water or when thrombin solutions are 

 added to it. Since the action of thrombin on fibrinogen is not 

 prevented by neutral salts in these concentrations, while an oxa- 

 lated plasma which clots readily on the addition of calcium is 

 prevented from so clotting when sodium chlorid or magnesium 

 sulphate is added previously to a concentration of 4 or 5 per cent., 

 it follows that in all probabihty the neutral salts exert a restraining 

 effect on the process of clotting because they prevent or retard the 

 activation of the prothrombin by the calcium. 



