#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, 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.