A Pacchionian body consists of a central core of subarachnoid tissue which is joined to the general subarachnoid tissue by a comparatively narrow stalk.

This is limited by a layer of arachnoid ; around which is a prolongation of the subdural space. Bounding this space is a very thin layer of membrane derived from the inner layer of the dura mater. It has been shown experimentally' that injections thrown into the subarachnoid space permeate the Pacchionian bodies and pass into the venous sinuses into which these bodies project. In this course the injection dis- tends the subdural space of the Pacchionian body, but does not enter the general subdural space. It should be mentioned, however, that anatomical pores by which the injection could pass do not exist. It is probable from these experiments that an outlet for the cerebrospinal fluid is provided by the Pacchionian bodies.