SUBARACHXOID SPACE AXD FLUID. 565 



in the cranium, the pia mater becomes continuous with the neurilemma. It 

 is supplied with nerves from the sympathetic. 



Towards the upper part of the cord, the pia mater presents a greyish 

 mottled appearance, which is owing to pigment particles deposited within 

 its tissue. 



THE ARACHNOID MEMBRANE. 



The arachnoid is a very fine delicate serous membrane, which, like other 

 membranes of thab class, forms the lining boundary of a shut sac. The 

 walls of this sac consist of two portions, one of which, a distinct membrane 

 on the surface of the pia mater, is the visceral or cerebral layer of the 

 arachnoid, while the other, giving the smooth surfaca presented by the dura 

 mater on its interior, is described by some anatomists as the parietal layer 

 of the arachnoid, while, according to the view taken by others, it is merely 

 the serous surface of the dura mater. 



The parietal wall of the arachnoid space is invested with a layer of polygonal 

 epithelial cells, which are flattened and nucleated. Besides this, it presents 

 in the greater part of its extent no tissue distinct from the dura mater ; and 

 hence it is that Koiliker and others object to the term parietal layer of 

 the arachnoid membrane as applied to the structure of this surface. It may 

 be mentioned, however, that in certain recesses, as for example at the sides 

 of the crista galli, and between the trabeculse into which the deep fibres of 

 the dura mater are thrown in the neighbourhood of the superior longitudinal 

 sinns, a small amount of delicate connective tissue beneath the epithelium 

 may be distinguished from the dense fibres of the dura mater. 



The visceral layer of the arachnoid is a distinct transparent membrane 

 which passes over the various eminences and depressions on the cerebrum 

 and cerebellum, without dipping into the sulci and smaller fissures ; nor is 

 it uniformly and closely adherent to the pia mater. The interval left 

 between the arachnoid membrane and pia mater is named generally the 

 subarachnoid space. 



This subarachnoid space is wider and more evident in some positions than 

 in others. Thus, in the longitudinal fissure, the arachnoid does not descend 

 to the bottom, but passes across, immediately below the edge of the falx, at 

 a little distance above the corpus callosum. In the interval thus left, the 

 arteries of the corpus callosum run backwards along that body. At the 

 base of the brain and in the spinal canal there is a wide interval between 

 the arachnoid and the pia mater. In the base of the brain, this subarachnoid 

 space extends in front over the pons and the interpeduncular recess as far 

 forwards as the optic nerves, and behind it forms a considerable interval 

 between the cerebellum and the back of the medulla oblongata. In the 

 spinal canal it surrounds the cord, forming a space of considerable extent. 



A certain quantity of fluid is contained between the arachnoid membrane 

 and the dura mater ; but it has been shown by Magendie that the chief part 

 of the cerebro-spinal fluid is lodged under the arachnoid, in the subarachnoid 

 space. 



Magendie also pointed out the existence of a sort of septum dividing the 

 spinal subarachnoid space at the back of the cord. This is a thin mem- 

 branous partition, which passes in the median plane from the pia mater 

 covering the posterior median fissure of the cord to the opposite part of the 

 loose portion of the arachnoid membrane. It is incomplete and cribriform ; 

 and consists of bundles of white fibres interlaced more or less with one 

 another. Fibrous bands of the same texture pass across the subarachnoid 



