AGRAY commissure referred to as the massa intermedia links the two thalami across the third ventricle. Such a massa can usually be seen clearly at the time of pneumography and appears as a round or oval shadow in lateral view. This shadow varies in size from case to case and may at times be so extensive as to simulate the filling defect caused by a third ventricle tumor. Until the publication of a recent paper by Davie and Baldwin (3), the radiological appearance of the massa intermedia had been given only fleeting attention in standard works (1, 2, 4–8). This paper presents a study of the size, site, and configuration of the massa intermedia on the basis of air ventriculography. Inferences about the actual extent of the structure could then be offered. Cases and Procedure Eight hundred and sixty patients were studied: 476 males and 384 females suffering from movement disorders as parkinsonism, dystonia musculorum deformans, intention tremor, or torticollis. None of the patients gave any evidence suggesti...