Detailed descriptions of the histologic features of vascular disturbances of the central nervous system are scarce. This may be because of a common misconception that many of the vascular changes observed in the central nervous system are "functional," and therefore "without anatomic basis." Of course what is meant is that a certain type of vascular alteration, though structural, may be reversible; the actual changes are often so minimal that it may be difficult to demonstrate them in fixed tissue. Any one, however, should recognize that a change in size of a vessel is a structural or a morphologic change. Possibly because of this loose concept of "functional" vascular changes, the histopathologically visible evidence of the alteration that remains is often overlooked; certainly, not much importance has been assigned to it. In this paper this particular vascular change will be described, and meaningless discussion as to whether it is "functional or