In the midline of the brain and within the confines of the corpus callosum either or both of the cavum septi pellucidi and cavum vergae are not infrequently found. Neither cavity has excited much interest either anatomically or clinically. The two cases here reported are, I believe, the first instances in which a diagnosis of these spaces, dilated in abnormal degree, has been made during life, the first in which the lesion has been found at or treated by operation and the first in which clinical symptoms are shown to be related to the lesions. But there is reason to believe that these cavities may not be uncommon and may not be unimportant in clinical neurology. Moreover, the diagnosis of the lesion is easy and unequivocal. The operative treatment is not difficult, though not unassociated with great danger if certain well defined precautions are not recognized. A permanent cure of