This is a frequent and serious lesion, occurring mostly as a result of embolism or thrombosis.

Embolism occurs in the great majority of cases in connection with old standing valvular disease of the heart, and especially in connection with disease of the mitral valve, associated with stenosis of that orifice and formation of thrombi in the left auricle. This condition of the valves is itself the result of a former endocarditis, and embolism of the cerebral vessels is specially liable to occur when valves, already the seat of sclerotic change, are again attacked by the inflammatory process. It may also happen in the recent acute forms, especially when of the malignant or ulcerative type, with thrombus formation on the denuded surface of the valves. In the former case the embolus results from the softening and breaking-down or detachment of the thrombi in the left auricle, or from the detachment of fragments of fibrine or calcareous matter from the indurated curtains at the mitral orifice. In the more acute forms of endocarditis the embolus may result from detachment of a fragment of fibrine from the inflamed mitral or aortic valve. So also, but much more rarely, may thrombi in the aorta become detached and carried off. Emboli come very exceptionally from the lungs, but sometimes in gangrene of the lungs there is a thrombosis of the veins, and from fragments of fibrine carried off we may have septic embolism of the brain. A cancer of the lungs after penetrating into the pulmonary vein may produce embolism.

It is matter of general observation that in cerebral embolism the Middle cerebral artery (or artery of the fissure of Sylvius) is the vessel plugged in the great majority of cases. It is also stated that the left middle cerebral is more frequently the seat of embolism than the right, but this has been doubted by competent observers. The frequency with which the middle cerebral is affected admits of easy •explanation. As the vertebral artery arises from the subclavian nearly at a right angle, it is not common for an embolus to pass into it. But the innominate and the carotids, being nearly in the direct line of the current from the aortic orifice, readily receive any fragment. Then again, the middle cerebral is the direct continuation of the internal carotid and an embolus will more readily be swept into it than diverge forward or backward. It is important here to bear in mind that the middle cerebral supplies the greater part of the basal ganglia, including nearly the whole of the corpus striatum and internal capsule and a part of the thalamus opticus. It also supplies the greater part of the motor convolutions.

Thrombosis of the cerebral arteries arises in consequence of some alteration of the walls of the arteries. It is predisposed to by weakness of the heart, but mere sluggishness of the circulation so induced will hardly cause thrombosis in a healthy artery, although it does so in the venous sinuses of the dura mater, where the circulation is naturally very much slower. Thrombosis is mostly induced by Atheroma of the arteries. The internal coat is thickened, and the surface is rough, while the calibre of the artery is already considerably encroached on, as Fig. 325 shows. Atheroma occurs mostly in old persons, and it is usually most pronounced in the larger arteries of the base, where there may be numerous yellow patches. But not infrequently it extends to the finer ramifications, and it is in these that it most readily leads to occlusion by thrombosis. The artery from which the figure is taken, for instance, had an external diameter of the eleventh of an inch. Whilst complete occlusion by thrombosis is unusual except in very small arteries, it may occur in those of larger size. In a case observed by the author one of the larger branches of the middle cerebral artery was thus affected, and an extensive softening in the temporo-sphenoidal lobe resulted. Syphilitic disease of the arteries is a much less frequent cause of thrombosis. It is met with in connection with gummata, and these may occur at any part of the surface of the brain. Thrombosis may also result around an embolus and extend backwards against the blood current. It may also occur in connection with various forms of aneurysm of the cerebral vessels.

Fig. 325. - Atheroma of a small cerebral artery the eleventh of an inch in diameter, with thrombosis. 6, atheromatous internal coat; c, thrombus on its surface partially organized. Between these two is an irregular clear space, which represents fresh blood, which had been insinuated between patch and thrombus, d, remaining calibre filled with blood, x 34.