THE CIRCULATING LIQUIDS OF THE BODY 37 



the corpuscles. For example, the blood of the horse clots 

 very slowly, and a low temperature lessens the rapidity of 

 coagulation of every kind of blood. If horse's blood is run 

 into a vessel surrounded by ice and allowed to stand, the 

 corpuscles, being of greater specific gravity than the plasma, 

 gradually sink to the bottom, and the clear straw-yellow 

 plasma can be pipetted off. Or, again, the addition of 

 neutral salts to blood may be used to delay coagulation, 

 the blood being run direct from the animal into, say, a 

 third of its volume of saturated magnesium sulphate 

 solution. The plasma may then be conveniently separated 

 from the corpuscles by means of a centrifugal machine. 

 Again, two ligatures may be placed on a large bloodvessel, 

 so that a portion of it can be excised full of blood and 

 suspended vertically; coagulation is long delayed, and the 

 corpuscles sink to the lower end. In these and many other 

 ways plasma free from corpuscles can be got ; and it is found 

 that when the conditions which restrain coagulation are 

 removed when, for instance, the temperature of the horse's 

 plasma is allowed to rise, or the magnesium sulphate plasma 

 is diluted with several times its bulk of water clotting takes- 

 place, with formation of fibrin in all respects similar to that 

 of ordinary blood-clot. The corpuscles themselves cannot 

 form a clot. From this we conclude that the essential 

 process in coagulation of the blood is the formation of 

 fibrin from some constituent of the plasma, and that the 

 presence of corpuscles in ordinary blood-clot is accidental. 

 In accordance with this conclusion, we find that lymph 

 entirely free from red corpuscles clots spontaneously, with 

 formation of fibrin ; and when fibrin is removed from newly- 

 shed blood by whipping it with a bundle of twigs or a piece 

 of wood, it will no longer coagulate, although all the cor- 

 puscles are still there. 



What, now, is the substance in the plasma which is 

 changed into fibrin when blood coagulates ? If plasma, 

 obtained in any of the ways described above, be saturated 

 with sodium chloride, a precipitate is thrown down. The 

 filtrate separated from this precipitate does not coagulate 

 on dilution with water ; but the precipitate itself the so- 



