THE ARACHNOID, 573 



brane. It passes over the various eminences and depressions on the 

 cerebrum and cerebellum, without dipping down into the sulci and 

 smaller fissures. Beneath it, between it and the pia mater, is a space 

 (subarachnoid space) in which is a considerable quantity of fluid (sub- 

 arachnoid fluid). 



The outer surface of the arachnoid is in contact with the smooth inner 

 surface of the dura mater, the epithelium upon which has been regarded 

 as a parietal layer of the arachnoid, closely united with the dura mater. 

 At certain recesses, and near the longitudinal sinus, a small amount of 

 loose connective tissue beneath the epithelium may be distinguished 

 from the dense fibres of the dura mater. AVith this epithelium the 

 arachnoid membrane (or visceral layer, as it was termed) has been 

 thought to be continuous at the various foramina, the two thus con- 

 stituting a closed sac similar to that of the pleura. But since the inner 

 surface of the dura mater in the greater part of its extent does not 

 present any distinct tissue which can be regarded as an attached mem- 

 brane, Kolliker, Henle, and others consider that there is no justification 

 for the assumption of a parietal layer of the arachnoid. Moreover 

 recent investigations into the relation between the arachnoid and dura 

 mater on the nerve roots as they leave the cranio-vertebral cavity have 

 shown that there is no such reflection of the arachnoid on to the dura 

 mater as had been supposed. (Axel Key and Retzius). 



The subarachnoid space is wider and more evident in some posi- 

 tions 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 ha,^e 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, being there of considerable extent. It is occupied, in both 

 brain and cord, by trabeculte and thin membranous extensions of 

 delicate connective tissue, connected on the one hand with the arachnoid, 

 and on the other with the pia mater. This tissue is most abundant 

 where the space between the two membranes is least. It is dense in 

 the neighbourhood of the vessels, and is continuous with the tissue of 

 their walls. 



The subarachnoid space communicates with the ventricles of the 

 brain by means of the foramen of Magendie, the opening into the 

 lower part of the fourth ventricle, through the membrane which closes 

 it (p. 513). Two other openings through this membrane exist, one 

 on each side, behind the upper roots of the glosso-phai-yngeal nerve 

 into the pouch-like extension of the membrane beneath *the flocculus 

 (Mierzejewsky). 



A certain quantity of fluid is contained between the arachnoid mem- 

 brane and tlie dura mater ; but it has been shown by Magendie that the 

 chief part of the cerebro-spinal fluid is lodged in the subarachnoid 

 space in the meshes of the trabecular tissue. 



The ligamentura denticulatum divides the spinal subaraclmoid space 

 into anterior and posterior portions. Magendie also pointed out the 



