AR ACHNOIDEA.
This delicate transparent membrane consists of connective tissue and is devoid of blood-vessels. It is separated from the dura by the subdural space and connected with the pia by strands of connective tissue. These strands form the subarachnoidal tissue, and the cleft between the arachnoid and the pia is the subarachnoidal space. The latter is traversed by the connective tissue trabeculw and plates of the subarachnoidal tissue and contains a fluid, the liquor cerebro-spinalis, in considerable quantity. The subarach noidal space communicates with the ventricles by means of the foramen Magendii and the foramina Luschkae (page 8o). Over the cerebral convolutions the subarachnoidal tissue is scanty, in these localities the arachnoid and the pia being fused into a common membrane. Over the cerebral fissures, on the contrary, the space between the two membranes is larger, since the pia penetrates into the fissures. The large spaces are found principally at the base of the brain and where the latter passes into the spinal cord ; in these locations, at certain places, the arachnoid is widely separated from the pia, resulting in the formation of the cisternae subarachnoideales. Such spaces are :
The cisterna cerebello-medullaris, between the posterior border of the cerebellum and the medulla oblongata ; The cisterna fossae Sylvii, over the Sylvian fossa ; The cisterna chiasmatis, surrounding the optic chiasm ; The cisterna interpeduncularis, between the cerebral crura ; The cisterna ambiens, ascending laterally from the cerebral peduncles to the corpora quadrigemina ; The cisterna corporis callosi, along the convex dorsal surface of the callosum.
In certain places, as on both sides of the sinus sagittalis superior or along the sinus transversus, villous projections from the outer surface of the arachnoid push before them the thin dura mater and encroach on the venous sinuses. Such elevations are called the arachnoidal villi or Pacchonian granulations (Fig. 9o). According to the investigations of Key and Retzius, these structures facilitate the passage of serous fluid into the venous spaces.