202 DISEASES OF THE BRAIN. 



arise, which proves that in these cases the endangered hemisphere 

 receives a sufficient amount of blood from collateral branches, particu- 

 larly through the circle of Willis. The reason for this difference is 

 not fully known. Hasse thinks that in the former case the continua- 

 tion of the thrombus from the point of ligation beyond the circle of 

 Willis prevents the formation of a collateral circulation. On closure 

 of the internal carotid or of the vertebral artery on one or both sides, 

 partial anaemia does not generally occur, since a collateral circulation 

 is usually quickly established through the circle of Willis. On the 

 other hand, closure of an artery originating above the circle of Willis, 

 e. g., the arteria fossae Sylvii, almost always induces anaemia of the 

 part supplied by the obstructed vessel, since here circumstances are 

 much more unfavorable for the formation of a collateral circulation. 



The pathological processes that chiefly induce closure of the arte- 

 ries of the brain are in some cases compression by tumors, but in most 

 cases obstruction by thrombosis formed at the spot or by emboli from 

 some other part. 



It is only exceptionally that the blood coagulates in cerebral ar- 

 teries whose walls are healthy (marasmic thrombosis). As a rule, the 

 thromboses form at points where, as a result of chronic endarteritis, or, 

 as is usually said, of atheromatous degeneration, the calibre of one of 

 the vessels of the brain is diminished and its inner wall roughened. 



The emboli by which cerebral vessels are closed are almost always 

 detached clots of fibrin, that have been deposited, in endocarditis or 

 valvular disease, on rough parts of the valves, or else portions of the 

 valves themselves, that have been washed off by the current of the 

 blood. These rarely come from necrotic points in the lungs, or from 

 thromboses of the pulmonary vein. In one very instructive case, pub- 

 lished by JSsmarch, an embolus obstructing the internal carotid came 

 from an aneurism of the carotid, having been set free by the manipu- 

 lations during an examination. 



We have but little to add in regard to the etiology of this form of 

 anaemia of the brain. Since the atheromatous affection, which most 

 frequently causes thrombosis of the cerebral vessels, generally occurs in 

 advanced age, the stoppage of the arteries of the brain by thromboses 

 is almost exclusively seen in aged persons, and plays an important 

 part in the diseases of old age. The case is different with embolism 

 of the cerebral arteries ; this is seen in young persons also, since en- 

 docarditis and valvular disease of the heart occur at all ages. 



The pathogeny of the partial necrosis, which results in many cases 

 of thrombosis and embolism of the cerebral arteries, is perfectly evi- 

 dent. This form of softening of the brain is analogous to the gangrene 

 of the extremities induced by closure of the vessels. In both cases 



