The Arachnoidea

The arachnoidea is a very delicate and transparent membrane situated between the dura and pia mater. Its outer surface forms the inner wall of the subdural space and is covered by a layer of endothelium continuous with that of the opposed surface of the dura mater. Between it and the pia mater is the subarachnoid space (Cavum subarachnoideale) which contains the cerebrospinal fluid. An inner surface can scarcely be said to exist, since deeply the membrane becomes a reticulum of fine fibers which traverse the subarachnoid space and are attached to the pia mater. The arachnoidea furnishes sheaths to the cerebral nerves from their superficial origins to a variable but usually short distance beyond the emergence from the dursal sac. In the case of the optic nerve this sheath extends to the eyeball.

The arachnoidea of the brain (Arachnoidea encephali), except in the case of the great longitudinal and transverse fissures, does not dip into the sulci on the surface of the brain. On the summits of the gyri it is so closely attached to the pia mater that the two form practically a single membrane. Its outer part bridges over the sulci, and here the subarachnoid space is partially divided up by the loose arachnoid tissue into intercommunicating cavities. In certain situations the arachnoidea is separated from the pia by spaces of considerable depth and extent. These enlargements of the subarachnoid space are termed subarachnoid cisterns (Cisternæ subarachnoidales). In them the subarachnoid tissue does not form a close network, but consists of a relatively small number of long, thread-like strands which traverse the cavity.

The chief cisternæ are: (1) the cisterna magna, which is at the angle formed between the posterior face of the cerebellum and the upper surface of the medulla oblongata. It communicates with the fourth ventricle through the foramen of Magendie (in man); and behind with the wide subarachnoid space of the spinal cord; (2) the cisterna pontis on the ventral surface of the pons; (3) the cisterna basalis, which lies at the base of the cerebrum and is divided by the optic chiasma into two parts (cisterna chiasmatis, cisterna interpeduncularis); (4) the cisterna fossæ lateralis, situated at the lower part of the lateral fissure (of Sylvius), and continuous with the cisterna basalis.

Along the upper border of the falx cerebri the arachnoidea bears bulbous excrescences, the arachnoid granulations or Pacchionian bodies. These are inclosed in thin evaginations of the dura mater and project into the sagittal sinus or the parasinoidal sinuses along either side of it. In some cases they are sufficiently large to exert pressure on the bone and produce in it depressions of variable depth.

The spinal arachnoidea (Arachnoidea spinalis) is directly continuous with that of the brain at the foramen magnum. It forms a relatively wide tube around the spinal cord, so that the latter (inclosed in the pia) is surrounded by a very considerable quantity of cerebrospinal fluid. The spinal subarachnoid space is traversed by fewer arachnoid trabeculæ than is the case in the cranium. It is partially subdivided by three imperfect septa. One of these, the septum dorsale, is median and dorsal. The other two, the ligamenta denticulata, are lateral and will be described with the pia mater.