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312 THE BLOOD.
in thin layers. If the haemoglobin is removed from the stroma and
dissolved by the blood liquid by any of the above-mentioned means
(see page 273), the blood becomes transparent and has then a " lake
color."
1
Less light is now reflected from its interior, and this laky
blood is therefore darker in thicker layers. On the addition of salt
solutions to the blood-corpuscles they shrink, more light is reflected,
and the coJor appears lighter. A great abundance of red corpuscles
makes the blood darker, while by diluting with serum or by a greater
abundance of white corpuscles the blood becomes lighter in appearance.
The different colors of arterial and of venous blood depend on the vary-
ing quantities of gas contained in these two varieties of blood, or, bet-
ter, on the different amounts of oxyhemoglobin and haemoglobin they
contain.
The most striking property of blood consists in its. coagulating within
a shorter or longer time, but as a rule very shortly after leaving the veins.
Different kinds of blood coagulate with varying rapidity; in human
blood the first marked sign of coagulation is seen in two to three minutes,
and within seven to eight minutes the blood is thoroughly converted into
a gelatinous mass. If the blood is allowed to coagulate slowly, the red
corpuscles have time to settle more or less before the coagulation, and
the blood-clot then shows an upper yellowish-gray or reddish-gray layer
consisting of fibrin enclosing chiefly colorless corpuscles. This layer
has been called crusta inflammatoria or phlogistica, because it has been
especially observed in inflammatory processes and is considered one
of the characteristics of them. This crustaf or " huffy coat,’" is not
characteristic of any special disease, and it occurs chiefly when the blood
coagulates slowly or when the blood-corpuscles settle more quickly
than usual. A buffy coat is often observed in the slowly coagulating
equine blood. The blood from the capillaries is not supposed to have the
power of coagulating.
Coagulation is retarded by cooling, by diminishing the oxygen, and by
increasing the amount of carbon dioxide, which is the reason that venous
blood and to a much higher degree blood after asphyxiation coagulates
more slowly than arterial blood. The coagulation may be retarded or
prevented by the addition of acids, alkalies, or ammonia, even in small
quantities; by concentrated solutions of neutral alkali salts and alkaline
earths, alkali oxalates and fluorides; also by egg-albumin, solutions of
sugar or gum, glycerin, or much water; also by receiving the blood in
oil. Coagulation may be prevented by the injection of a proteose solu-
tion or of an infusion of the leech into the circulating blood, but this
1
R. Du Bois-Reymond presents objections to the general use of the above terms,
in Centralbl. f. Physiol., 19, p. 65.
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