d formed
in the interior of the heart or of a blood vessel, and the process by
which such a clot forms is known as _thrombosis_. It would appear that
slowing or stagnation of the blood-stream, and interference with the
integrity of the lining membrane of the vessel wall, are the most
important factors determining the formation of the clot. Alterations in
the blood itself, such as occur, for example, in certain toxaemias, also
favour coagulation. When the thrombus is formed slowly, it consists of
white blood cells with a small proportion of fibrin, and, being
deposited in successive layers, has a distinctly laminated appearance on
section. It is known as a _white thrombus_ or laminated clot, and is
often met with in the sac of an aneurysm (Fig. 72). When rapidly formed
in a vessel in which the blood is almost stagnant--as, for example, in a
pouched varicose vein--the blood coagulates _en masse_, and the clot
consists of all the elements of the blood, constituting a _red thrombus_
(Fig. 66). Sometimes the thrombus is _mixed_--a red thrombus being
deposited on a white one, it may be in alternate layers.

When aseptic, a thrombus may become detached and be carried off in the
blood-stream as an embolus; it may become organised; or it may
degenerate and undergo calcification. Occasionally a small thrombus
situated behind a valve in a varicose vein or in the terminal end of a
dilated vein--for example in a pile--undergoes calcification, and is
then spoken of as a _phlebolith_; it gives a shadow with the X-rays.

When infected with pyogenic bacteria, the thrombus becomes converted
into pus and a localised abscess forms; or portions of the thrombus may
be carried as emboli in the circulation to distant parts, where they
give rise to secondary foci of suppuration--pyaemic abscesses.

#Embolism.#--The term _embolus_ is applied to any body carried along in
the circulation and ultimately becoming impacted in a blood vessel. This
occurrence is known as _embolism_. The commonest forms of embolus are
portions of thrombi or of fibrinous formations on the valves of the
heart, the latter being usually infected with micro-organisms.

Embolism plays an important part in determining one form of gangrene, as
has already been described. Infective emboli are the direct cause of the
secondary abscesses that occur in pyaemia; and they are sometimes
responsible for the formation of aneurysm.

Portions of malignant tumours also may form emboli,