752 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



Cranial or Encephalic Arachnoid. — There is nothing special to note in 

 the parietal layer. 



If the visceral layer be traced from the occipital foramen — where it is 

 continuous with the spinal arachnoid — to the anterior extremity of the cerebral 

 lobes, it is seen to be prolonged inferiorly on the lower face of the medulla 

 oblongata, as far as the pituitary stem, to which it furnishes a covering : the 

 pituitary gland itself is not covered by the arachnoid, except on a portion of the 

 superior or deep face ; from the medulla oblongata it is carried forward, and 

 extends on each side of the cerebellum and cerebral lobes. Superiorly, this 

 internal layer spreads over the surface of the cerebellum, and is reflected at the 

 bottom of the Assure between that organ and the cerebral hemispheres, over the 

 posterior extremity of the latter, enveloping them separately by descending into 

 the interlobular fissure as far as the corpus callosum. Reaching the anterior 

 extremity of the cerebrum, it gains the olfactory lobes, is principally prolonged 

 on their supero-posterior face, and doubles around the ganghon of grey substance 

 on their inferior face, to be continued with the parietal layer. 



In covering the external surface of the brain, the cranial arachnoid does not 

 adhere everywhere to the nerve substance, but is only slightly connected with it, 

 through the medium of the pia mater, at such salient points as the summits of 

 the cerebral convolutions. Neither does it dip down to enter the sulci existing 

 between these parts, but passes over them, and in this way forms a large number 

 of sub-arachnoid spaces {epi-cerehral spaces) analogous to that developed over the 

 whole extent of the spinal cord. 



These spaces, which are filled by the sub-arachnoid fluid, differ widely in 

 form and dimensions. In Man, three principal have been described, and these 

 are also found in animals ; Magendie has named them the confluents of the sub- 

 arachnoid fluid. Of these three confluents, the anterior is situated in advance of 

 the chiasma of the optic nerves, between the two cerebral lobes ; the inferior, 

 the largest, is comprised between the pituitary stem and the pons Varolii to the 

 surface of the peduncles of the cerebrum ; while the third, or posterior confluent, 

 lies behind the cerebellum, at the calamus scriptorius. 



None of these spaces communicate with the internal cavities of the brain, 

 and, consequently, the sub-arachnoid fluid cannot enter them. Magendie has 

 nevertheless described a communication between the posterior confluent and the 

 ventricle of the cerebellum -, though the opening he described towards the 

 calamus scriptorius has not been found in the Horse by Renault, and we may 

 affirm, with Lavocat, that it does not exist in the other animals. 



The Sub-arachnoid Fluid. — The fluid contained in the sub-arachnoid 

 spaces is slightly yellow or colourless, and perfectly limpid and transparent. 

 Some authorities assert that it is secreted by the visceral layer of the arachnoid, 

 and others by the pia mater. According to the remark made by Cruveilhier, the 

 nerve-centres are immersed in it, like a foetus in the liquor amnii ; and this 

 remark, which is particularly applicable to the spinal cord, gives the key to the 

 use of this fluid, which keeps the organ away from the walls of the spinal canal, 

 deprives it of the greater part of its weight (Foltz). and thus diminishes every 

 kind of injury to which it might be exposed — whether from without (concussion), 

 or within (intermittent afflux of blood) — in a word, the cerebro-spinal fluid 

 constantly maintains an equilibrium in pressure around the central nervous axis. 



(This fluid, so necessary for the support and protection of the cord and brain, 

 is alkaline, and contains but a small quantity of albumen ; it varies in quantity. 



