THE CIRCULATING LIQUIDS 'OF THE BODY 31 



formed by a change in some constituent or constituents of the 

 normal blood. Now, it has been shown that there exists in the 

 plasma the liquid portion of unclotted blood a substance from 

 which fibrin can be derived, while no such substance is present 

 in the corpuscles. In various ways coagulation can be prevented 

 or delayed, and the plasma separated from 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 

 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 

 (the so-called experiment of the ' living test-tube ') ; coagula- 

 tion 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 corpuscles 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, 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-called plasmine of Denis on being 



* Bird's corpuscles, however, washed free from plasma, will form a clot 

 when laked in various ways, as by addition of water or by freezing and 

 thawing. 



