#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 pyæmia; 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 hæmorrhage. 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.