ie 
spinal cord. ‘This arachnoidal sac is most capacious towards its lower part, where it 
envelopes the lower end of the cord and the collection of long nerve-roots which 
constitute the cauda equina. 
As the nerves, both from the brain and the cord, pass outwards they receive an 
investment from the arachnoid, which runs for a short distance upon them and 
then comes to an end. 
Subarachnoid Space (cavum subarachnoidale)—The interval between the 
arachnoidea and the pia mater receives the name of the subarachnoid space. It 
contains the cerebro-spinal fluid, and communicates freely through certain well- 
defined apertures with the ventricular cavities in the interior of the brain. Three 
of these (viz. the foramen of Majendie and another at the extremity of each lateral 
recess) are in connexion with the fourth ventricle; two are slit-like openings into 
the lateral ventricles, and are placed at the extremity of each descending horn. 
Within the cranium the subarachnoid space is broken up by a meshwork of fine 
filaments and trabecule, which connects the two bounding membranes (viz. the 
arachnoidea and the pia mater) in the most intimate manner, and forms a delicate 
sponge-like interlacement between them. Where the arachnoidea passes over the 
summit of a cerebral convolution, and is consequently closely applied to the sub- 
jacent pia mater, the meshwork is so dense and the trabecule so short that it is 
hardly possible to discriminate between the two membranes. To all intents and 
purposes they form in these localities one lamina. In the intervals between the 
rounded margins of adjoining convolutions, however, distinct angular spaces exist, 
where the subarachnoid trabecular tissue can be studied to great advantage. These 
intervals on the surface of the cerebrum constitute numerous communicating 
channels which serve for the free passage of the subarachnoid fluid from one part of 
the brain to another. The larger branches of the arteries and veins of the brain 
traverse the subarachnoid space; their walls are directly connected with the sub- 
arachnoid trabecule, and are bathed by subarachnoid fluid. 
In certain situations within the cranium the arachnoidea is separated from the 
pla mater by intervals of considerable width and extent. These expanded portions 
of the subarachnoid space are termed cisterne subarachnoidales. In these the sub- 
arachnoid tissue is much reduced. There is no longer a close meshwork; the 
trabecule connecting the two bounding membranes take the form of long fila- 
mentous intersecting threads which traverse the spaces. All the subarachnoid 
cisterns communicate in the freest manner with each other and also with the 
narrow Channels on the surface of the cerebrum. 
560 THE NERVOUS SYSTEM. 
Certain of these cisterns require special mention. The largest and most conspicuous is the 
cisterna magna. It is formed by the arachnoid membrane bridging over the wide interval 
between the back part of the under surface of the cerebellum and the medulla. It is continuous 
through the foramen magnum with the posterior part of the wide subarachnoid space of the cord. 
The cisterna pontis is the continuation upwards on the floor of the cranium of the anterior 
part of the subarachnoid space of the spinal cord. In the region of the medulla it is continuous 
behind with the cisterna magna, so that this subdivision of the brain, like the spinal cord, is 
surrounded by a wide subarachnoid space. 
In front of the pons Varolii the arachnoidea bridges across between the projecting temporal 
lobes, and covers in the deep hollow in this region of the brain. This space is called the cisterna 
basalis, and within it are placed the large arteries which take part in the formation of the circle 
of Willis. Leading out from the cisterna basalis there are certain wide subarachnoid channels. 
Two of these are prolonged into the Sylvian fissures, and in these are accommodated the middle 
cerebral arteries. Anteriorly the basal cistern passes into a space in front of the optic chiasma, 
and from this it is continued into the great longitudinal fissure above the corpus callosum. In 
this subarachnoid passage the anterior cerebral arteries are lodged. 
The spinal part of the subarachnoid space is a very wide interval which is 
partially subdivided into compartments by three incomplete septa. One of these 
is a mesial partition called the septum posticum, which connects the pia mater 
covering the posterior aspect of the cord with the arachnoid mater. In the upper 
part of the cervical region the septum posticum is imperfect, and is merely repre- 
sented by some strands passing between the two membranes; in the lower part of 
the cervical region and in the dorsal region it becomes tolerably complete. The 
other two septa are formed by the ligamenta denticulata which spread outwards 
from either side of the spinal cord. These will be described with the pia mater. 
