INTRODUCTION After a cranial trauma, blood, cerebrospinal fluid or a serous effusion may accumulate underneath the dura and may, after a shorter or longer latent period, give rise to symptoms of disturbed function of the brain. The clinical pictures presented by these patients are often characteristic and similar, but insufficient attention has been directed to the variety of the lesions that may be found in the subdural space. 1. There may be a collection of fluid or clotted blood underneath the dura. This variety is the chronic subdural hematoma recently described by Putnam and Cushing. The blood usually overlies one hemisphere, although it may be found on both sides, and results in more or less marked compression of part of the brain. The symptoms are promptly relieved after fluid blood has been evacuated or after large blood clots have been removed 2. Turbid or blood stained cerebrospinal fluid or a