ame implies, the chief
physiological property of fibrinogen is that it can give rise to fibrin,
the solid substance formed when blood clots. It possesses the typical
properties of a globulin, i.e. it coagulates on heating (in this
instance at a temperature of 56 deg.C.), and is precipitated by half
saturating its solution with ammonium sulphate. It differs from other
globulins in that it is less soluble. It is only present in very small
quantities, 0.4%. The other globulin, serum-globulin, is not coagulated
until 75 deg.C. is reached, and we now know that it is in reality a
mixture of several proteins, but so far these have not been completely
separated from one another and obtained in a pure form. On dialysing a
solution of serum-globulin a part is precipitated, and this portion has
been termed the eu-globulin fraction, the remainder being known, in
contradistinction, as the pseudo-globulin. Again, on diluting a solution
and adding a small amount of acetic acid a precipitate is formed which
in some respects differs from the remainder of the globulin present.
Whether in these two instances we are dealing with approximately pure
substances is extremely doubtful. A further important point in connexion
with the chemistry of the globulins is that dextrose may be found among
their decomposition products, i.e. that a part of it, or possibly the
whole, possesses a glucoside character.

Serum-albumin gives all the typical colour and precipitation reactions
of the albumins. If plasma be weakly acidified with sulphuric acid, then
treated with crystals of ammonium sulphate until a slight precipitate
forms, filtered and the filtrate allowed to evaporate very slowly,
typical crystals of serum-albumin may form. According to many it is a
uniform and specific substance, but others hold the view that it
consists of at least three distinct substances, as shown by the fact
that if a solution be gradually heated coagulation will occur at three
different temperatures, viz. at 73 deg., 77 deg. and 84 deg. C. On the
other hand the close agreement between different analyses of even the
amorphous preparations points to there being but one serum-albumin.

When blood clots two new proteins make their appearance in the fluid
part of the blood, or serum, as it is now called. The first of these is
fibrin ferment (for its origin see section on _Clotting_ below). The
other, fibrinoglobulin, possesses all the typical characteristics of the
globulins a