146 THE BLOOD. 



tion, by contact with foreign matter, by moderate dilution with water, 

 by addition of calcium salts, and by fibrin ferment and nucleo-proteids. 

 It is delayed by cold, by dilution with solutions of neutral salts 

 or of sugar, by intravenous injection of albumose, and of various 

 other organic substances, such as diastatic ferments ; also by prevention 

 of contact with foreign matter, as by drawing it into oil. It is also 

 prevented if the soluble lime salts are precipitated by soluble oxalates, 

 by fluorides, or by soap. A temperature of 56 C. prevents coagulation 

 by precipitating the fibrinogen upon which the coagulation depends. 

 It remains fluid for an indefinite time within the living blood vessels, 

 even in a portion of vessel which has been isolated by ligatures. But if 

 the inner surface of any blood vessel is injured, the blood tends to 

 deposit a coagulum upon the injured part. And if a foreign substance 

 is introduced into a blood vessel a clot forms upon it. 



It also coagulates within the vessels of a living animal if a solution 

 of nucleo-proteids is injected in a certain amount into the veins (Wool- 

 dridge) ; l but if the amount injected is too small to cause coagulation, the 

 opposite effect is obtained, the coagulability being temporarily destroyed 

 (negative phase, Wooldridge). These effects are not peculiar to nucleo- 

 proteids, but have been shown to be also produced by intravenous injection 

 of artificially prepared " colloids " 2 (see p. 37), and by snake-venom. 3 



If the coagulation is prevented by any of the above means, the cor- 

 puscles, which are heavier than the plasma, tend to fall to the bottom of 

 the vessel, and to leave the upper layers of plasma clear. At the junction 

 between the mass of subsided red corpuscles and the plasma is a " buffy " 

 layer containing most of the white corpuscles. The subsidence may be 

 accelerated by centrifugalising the blood. If cold be used to delay the 

 coagulation, or if the blood be contained in a ligatured vein, carefully 

 removed from an animal immediately after death, and suspended in a glass 

 vessel, pure plasma may be drawn off from the upper layer completely free 

 from red corpuscles, but usually containing a few leucocytes. The experi- 

 ment is best performed with horse's blood, the corpuscles being relatively 

 heavier in this as compared with that of other animals. This plasma 

 clots on "being placed in a glass vessel at the temperature of the air, but 

 much more slowly than a sample of the original blood, and the more 

 slowly the fewer the blood platelets and leucocytes it contains. If a 

 sample be taken from the buffy layer containing, therefore, many leuco- 

 cytes and many blood platelets the clotting is speedy and firm. If 

 bird's blood is rapidly and repeatedly centrif ugalised, plasma is obtainable 

 almost entirely free from corpuscles, and no clotting occurs in it for 

 days on standing in a glass vessel. 4 It appears, therefore, that the 

 coagulation is independent of the red corpuscles, and is dependent 

 upon the plasma and white corpuscles, and perhaps also upon the 

 blood platelets. It is also dependent upon the presence of calcium 

 salts. The exact relations which these factors bear to one another in 

 the phenomenon of coagulation will be discussed later in considering the 

 properties of fibrinogen. 



The delay of coagulation produced by neutral salts is best obtained 



1 Proc. Hoy. Soc. London, 1886, vol. xviii. p. 186 ; Arch. f. PhysioL, Leipzig, 1886, 

 S. 397. 



2 Pickering, Journ. PhysioL, Cambridge and London, 1895, vol. xvii. (Proc. PhysioL 

 Soc., p. v); and Halliburton and Pickering, ibid., 1895, vol. xviii. p. 285. 



3 C. J. Martin, Journ. and Proc. Eoy. Soc. New South Wales, Sydney, 1895. 



4 Delezenne, Compt. rend. Soc. de. bioL, Paris, 1896, p. 782. 



