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 " huffy " 
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 huffy 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. Roy. 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. 2S5. 
3 C. J. Martin, Journ. and Proc. Boy. Soc. New South Wales, Sydney, 1895. 
4 Delezenne, Compt. rend. Soc. de. biol., Paris, 1896, p. 782. 
