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Among the more unusual neurologic conditions which may concern the otologist are certain abnormal accumulations of cerebrospinal fluid variously termed "meningitis serosa circumscripta,"1 "meningeal hydrops," 2 "otitic hydrocephalus" 3 and "arachnoiditis." 4 The cases recorded in the literature, although showing considerable variation as to the origin, course and symptoms of the condition, nevertheless have certain points in common. The clinical picture as a rule suggests some serious intracranial condition or complication, demanding surgical treatment. There is uniformly evidence of increased pressure; the cerebrospinal fluid either is normal or shows only a mild meningitic reaction, while the simple relief of pressure either by operation or through other means generally effects a rapid and permanent cure. While the pathogenesis is by no means clear in every case, the majority of those observations which were made at operation or after histologic study indicated changes in the arachnoid membrane. The term arachnoiditis will therefore be used as a general name for the conditions described. ANATOMIC AND PHYSIOLOGIC CONSIDERATIONSThe Arachnoid Membrane.-It will be recalled that the meninges of the brain consist of three layers : the dura mater, or outermost covering, which is closely adherent to the inner surface of the cranial bones and is also reflected to form certain rigid septums which project into the cranial cavity; the arachnoid membrane, separated from the dura mater by a potential cavity known as the subdural space, and the innermost layer or pia mater, which is closely applied to the surface of the brain and follows its every fold and convolution. The arachnoid is a delicate

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