A system of practical medicine. By American authors. Vol. 5

by Unknown

The sources whence cerebral emboli may spring are various, but cannot be found outside a certain range. They may, in the first place, be torn off from vegetations upon either the mitral or aortic valves; and this source is probably the most common. The appendix of the left auricle may furnish a plug from the thrombi formed among its trabeculæ, or the aorta from an aneurism or from parietal thrombi formed, upon spots roughened by atheroma. The pulmonary veins are occasionally the source of the embolus, though this is not very common.

It is rather doubtful whether an embolus can find its way from the systemic veins through the lungs to the brain, but it is possible that small emboli may do so, and increase in size from the addition of fresh fibrin when floating in the blood-current. The occurrence of pyæmic abscesses in the brain would suggest the possibility of this, though it is, on the other hand, possible that the brain abscesses are secondary to older ones in the lungs. In some cases, however, a careful examination does not disclose the source of the embolus.

In the blood-current the embolus may give rise to no symptoms whatever, and even after its arrival in the cerebral circulation it may lodge in such a way as not entirely to obstruct the current. In most instances, however, it does not stop until it plugs the vessel completely and arrests the current of blood beyond it for a moment. Whether it shall completely deprive the portion of brain to which it is distributed depends upon its situation as regards anastomoses and upon the formation of secondary thrombus. Hence the knowledge of the distribution of the arteries supplying the brain—that is, the two carotids and two vertebrals—is of more importance in reference to embolism and thrombosis than to cerebral hemorrhage, where the effusion takes place from quite small branches.

The anterior portion of the brain, including the anterior and posterior central convolutions and the first temporal, are supplied with blood by the two terminal branches of the internal carotid, the anterior and middle cerebral, the ganglia underlying these portions of the cortex being supplied, as already stated, by small branches arising near the origin of these two trunks, and principally the second. The anterior cerebrals of the two sides are connected by the anterior communicating, which is a short and usually wide vessel. Sometimes one anterior cerebral branches in the longitudinal fissure, and supplies a part of both sides. Hence in plugging of one internal carotid which does not reach its bifurcation a collateral supply may be received from the other side. If, however, an embolus or thrombus has penetrated beyond the origin of the middle cerebral, this vessel can no longer receive a supply from the anterior.

The posterior communicating arteries are two small vessels which connect on each side the posterior cerebrals and either the carotid, just as it gives off its two chief cerebral branches, or else the middle cerebral close to its origin. These arteries may be of quite unequal size, that upon the right usually being the larger, and sometimes so large as to give the appearance of being the principal origin of the posterior cerebral. When this happens the part of the posterior cerebral which arises from the basilar may be reduced to a minute arteriole, and the basilar, almost entire, goes to supply the left side of the brain. This condition of the posterior communicating may exist to some extent on both sides in the same brain. It is probable that in many cases these arteries are too small to be of great value in re-establishing the circulation in the anterior portion of the brain when it is suddenly interrupted by an embolus.

When the large trunks leave the circle of Willis to be distributed upon the surface of the brain, after giving off from the first centimeter or two of their course the nutrient arteries for the deep-seated ganglia, they break up into several branches which ramify upon the surface, but, as Duret has shown, undergo very few anastomoses. Instead of forming, as was once supposed, a richly inosculating network, small branches penetrate into the brain-substance perpendicularly from the superficial vessels, but these do not communicate freely with each other by vessels larger than capillaries.

From these anatomical conditions it happens that when a vascular territory is deprived of its normal supply by an embolus, it cannot be supplied with blood from surrounding districts. A certain limited amount of collateral supply is possible through the capillaries and the rare anastomoses, but it is only around the edges, and the centre of the territory becomes destitute of circulating blood. Thus an embolus does not in the brain produce, as it does in other organs with more abundant collateral supply, a large hemorrhagic infarction.

Small hemorrhages may, however, take place around the edges of the softening, and when a number of small emboli are present, so as to afford a number of overlapping areas with their borders of congestion, a red softening may be the result. When the emboli are very small, and at the same time not numerous enough to occlude all the ultimate ramifications of a trunk, the vascular compensation may be rapidly completed.

The change produced in the cerebral substance from cutting off its supply of blood is known as anæmic necrosis, and includes what has been known as white softening, with probably some yellow, and possibly a little red softening, the latter in case where simple softening has been complicated by hemorrhage.