664 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



surface of the spinal cord by thin cellular filaments detached from the pia 

 mater. 



CRANIAL OB 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 

 isthmus, as far as the pituitary stalk, to which it furnishes a sheath : the 

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

 the superior or deep face ; from the isthmus 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 fissure 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. 

 Beaching the anterior extremity of the cerebrum, it gains the olfactory 

 lobes, is principally prolonged on their supero-posterior face, and doubles 

 around the ganglion of grey substance on their inferior face, to be con- 

 tinued with the parietal layer. 



In covering the external surface of the encephalon, the cranial arachnoid 

 does not adhere everywhere to the nervous substance, but is only slightly 

 connected with it, through the medium of the pia mater, at such salient 

 portions 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 subarachnoid spaces analogous to 

 that developed over the whole extent of the spinal cord. 



These spaces, which are filled by the subarachnoid 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 subarachnoid 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 stalk and the 

 annular protuberance to the surface of the pedunculi 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 en- 

 cephalon, and, consequently, the subarachnoid fluid cannot enter them. 

 Magendie has nevertheless described a communication between the pos- 

 terior confluent and the ventricle of the cerebellum ; though the opening he 

 described towards the calamus scriptorius has not been found in the Horse 

 by M. Renault, and we believe we may affirm, with M. Lavocat, that it does 

 not exist in the other animals. 



THE SUBARACHNOID FLUID. The fluid contained in the subarachnoid 

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

 Some authorities admit that it is secreted by the visceral layer of the 

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

 Gruveilhier, the nervous 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 concussion to which 

 it might be exposed. 



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



