and their impaction
in the vessels may lead to the development of secondary growths in
distant parts of the body.

Fat and air embolism have already been referred to.

ARTERITIS

_Pyogenic._--Non-suppurative inflammation of the coats of an artery may
so soften the wall of the vessel as to lead to aneurysmal dilatation. It
is not uncommon in children, and explains the occurrence of aneurysm in
young subjects.

When suppuration occurs, the vessel wall becomes disintegrated and gives
way, leading to secondary haemorrhage. If the vessel ruptures into an
abscess cavity, dangerous bleeding may occur when the abscess bursts or
is opened.

_Syphilitic._--The inflammation associated with syphilis results in
thickening of the tunica intima, whereby the lumen of the vessel becomes
narrowed, or even obliterated--_endarteritis obliterans_. The middle
coat usually escapes, but the tunica externa is generally thickened.
These changes cause serious interference with the nutrition of the parts
supplied by the affected arteries. In large trunks, by diminishing the
elasticity of the vessel wall, they are liable to lead to the formation
of aneurysm.

Changes in the arterial walls closely resembling those of syphilitic
arteritis are sometimes met with in _tuberculous_ lesions.

#Arterio-sclerosis# or #Chronic Arteritis#.--These terms are applied to
certain changes which result in narrowing of the lumen and loss of
elasticity in the arteries. The condition may affect the whole vascular
system or may be confined to particular areas. In the smaller arteries
there is more or less uniform thickening of the tunica intima from
proliferation of the endothelium and increase in the connective tissue
in the elastic lamina--a form of obliterative endarteritis. The
narrowing of the vessels may be sufficient to determine gangrene in the
extremities. In course of time, particularly in the larger arteries,
this new tissue undergoes degeneration, at first of a fatty nature, but
progressing in the direction of calcification, and this is followed by
the deposit of lime salts in the young connective tissue and the
formation of calcareous plates or rings over a considerable area of the
vessel wall. To this stage in the process the term _atheroma_ is
applied. The endothelium over these plates often disappears, leaving
them exposed to the blood-stream.

Changes of a similar kind sometimes occur in the middle coat, the lime
salts being deposited amon