100 THE BLOOD AND THE LYMPH 



transfuse blood without making a vessel-to-vessel anastomosis. (3) Mix- 

 ing the blood with chemicals that are capable of removing the calcium 

 from solution. Such reagents are potassium or sodium oxalate (in a con- 

 centration of 0.1 per cent after mixing), and sodium fluoride and sodium 

 citrate (2 per cent solution, with one part of the solution to four parts 

 of blood). (4) Mixing the blood with certain neutral salts, particularly 

 the sulphates of sodium and magnesium (one part of 27 per cent solution 

 of magnesium sulphate mixed with four parts of blood). Blood thus 

 treated is known as "salted blood," and the plasma separated by centri- 

 fuging, as "salted plasma." Clotting is readily induced by adding water 

 to the salted blood or plasma, and in this way diminishing the concen- 

 tration of the salts. (5) The addition to blood of one of a class of sub- 

 stances known as antithrombins. Leech extract or the purified substance 

 separated from it, know r n under the trade name of "hirudin," and sub- 

 stances present in blood removed from animals after they have been 

 injected with peptone solutions, are examples. 



The methods which have just been described are those applied to blood 

 after it has left the blood vessels. Another interesting group of anti- 

 coagulants prevent clotting only when injected into the blood vessels of 

 the living animal. The most powerful example of this group is snake 

 venom, certain varieties of which can prevent clotting in the dosage of 

 %QO f a milligram for each kilogram of body weight. Similar but much 

 less potent effects are produced by the injection of several proteolytic 

 enzymes, but most attention has been paid to the effect of commercial 

 peptone injected in solution intravenously in the proportion of 0.3 gram 

 to each kilogram of body weight. Blood subsequently removed up to about 

 half an hour or more does not clot, and as we have already seen, if added 

 to blood from another animal, materially retards clotting. This group of 

 intra vitam anticoagulants is particularly interesting, since none of the 

 substances belonging to it is capable of preventing clotting of blood 

 when mixed with this after it has been shed. Their action therefore 

 obviously depends on the production of some substance in the body, 

 probably, as we shall see later, in the liver, since they fail to act after 

 the removal of this organ from the circulation (see page 111). 



The time of clotting varies greatly according to the conditions under 

 which the blood is collected and the animal from which it is derived. 

 Human blood, for example, received into a test tube from a puncture 

 through the skin may clot at any time within three or ten minutes, five 

 minutes being taken as an average time for blood kept at a temperature 

 of about 20 C. This time may be considerably shortened by increasing 

 the extent of foreign material with which the blood comes into contact, 

 and more particularly by whipping the blood with a bunch of twigs or 



