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.