It is a matter of common observation that the cerebral hemisphere, that is, the seat of a new growth, is usually larger than its fellow of the opposite side. This enlargement is due to several factors, the first of which is the presence of the tumor in the involved hemisphere. In the second place, there are vascular disturbances which may account for some of the discrepancy. Edema, thrombosis and hemorrhage are all known to occur within or in the vicinity of a tumor. Reactive gliosis also plays a part in the notable enlargement of the involved hemisphere. There are cases in which the tumor is of small size, compact, sometimes even calcified, in which, in spite of the absence of vascular disturbances, the hemisphere in which it is situated is enlarged out of all proportion to the size of the neoplasm. It is in these cases that reactive gliosis is