422 BLOOD AND LYMPH. 



the onset of clotting. This is easily understood when it is remem- 

 bered that the breaking down of leucocytes and blood plates is 

 hastened by contact with foreign surfaces. It has been proposed 

 also to hasten clotting in case of hemorrhage by the use of ferment 

 solutions. Hot sponges or cloths applied to a wound hasten 

 clotting, probably by accelerating the formation of ferment and the 

 chemical changes of clotting. Coagulation may be retarded 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 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 centri- 

 fugalized, 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 coag- 

 ulation, for example, in testing the efficacy of a given ferment 

 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 ferment solutions are added to it. How the salts prevent 

 coagulation is not definitely known possibly by preventing the 

 disintegration of corpuscles and the formation of ferment, possibly 

 by altering the chemical properties of the proteids. 



3. By the Action of Oxalate Solutions. If blood as it flows from 

 the vessels is mixed with solutions of potassium or sodium oxalate 

 in proportion sufficient to make a total strength of 0.1 per cent, 

 or more of these salts, coagulation is prevented entirely. Ad- 

 dition of an excess of water does not produce clotting in this case, 

 but solutions of some soluble calcium salt quickly start the process. 

 The explanation of the action of the oxalate solutions is simple: 

 they are supposed to precipitate the calcium as insoluble calcium 

 oxalate. 



4. By the Action of Sodium Fluorid. Blood drawn directly into 

 a solution of sodium fluorid (1 part of a 3 per cent, solution of 

 sodium fluorid to 9 parts of blood) does not clot. Addition of 



