ondition, such as occurs when it escapes from the blood vessels, it
undergoes a peculiar change known as _coagulation_.(11) In this change the
corpuscles are collected into a solid mass, known as the _clot_, thereby
separating from a liquid called the _serum_. The serum, which is similar
in appearance to the blood plasma, differs from that liquid in one
important respect as explained below.

*Causes of Coagulation.*--Although coagulation affects all parts of the
blood, only one of its constituents is found in reality to coagulate. This
is the fibrinogen. The formation of the clot and the separation of the
serum is due almost entirely to the action of this substance. Fibrinogen
is for this reason called the _coagulable constituent of the blood_. In
the plasma the fibrinogen is in a liquid form; but during coagulation it
changes into a white, stringy solid, called _fibrin_. This appears in the
clot and is the cause of its formation. Forming as a network of
exceedingly fine and very delicate threads (Fig. 11) _throughout the mass
of blood_ that is coagulating, the fibrin first entangles the corpuscles
and then, by contracting, draws them into the solid mass or clot.(12) The
contracting of the fibrin also squeezes out the serum. This liquid
contains all the constituents of the plasma except the fibrinogen.

[Fig. 11]

Fig. 11--*Fibrin threads* (after Ranvier). These by contracting draw the
corpuscles together and form the clot.

*Fibrin Ferment and Calcium.*--Most difficult of all to answer have been
the questions: What causes the blood to coagulate outside of the blood
vessels and what prevents its coagulation inside of these vessels? The
best explanation offered as yet upon this point is as follows: Fibrinogen
does not of itself change into fibrin, but is made to undergo this change
by the presence of another substance, called _fibrin ferment_. This
substance is not a regular constituent of the blood, but is formed as
occasion requires. It is supposed to result from the breaking down of the
white corpuscles, and perhaps also from the blood platelets, when the
blood is exposed to unnatural conditions. The formation of the ferment
leads in turn to the changing of the fibrinogen into fibrin.

Another substance which is necessary to the process of coagulation is the
element calcium. If compounds of calcium are absent from the blood,
coagulation does not take place. The