We can thus demonstrate the existence of a fluid, which during life and in a state of health occupies the sub-arachnoid cavity and main tains the two layers of the arachnoid membrane in contact with each other. This fluid is designated by Majendie the , cerebro-spinal fluid.
The first distinct recognition of this fluid in its proper locality is due to Cotunnius, who stated the results of his observations in his memoir " de Ischiade Nervosft," preserved in Sandifort's collection of dissertations. Cotun nius was led to the discovery by remarking the great disproportion between the dimensions of the spinal canal and the bulk of its contents, so that a considerable interval exists between the internal suiface of the former and the spinal cord, which must be filled by something ; and lie attributes its having been so completely overlooked by previous anatomists to the fa shion of opening the head before the spine, which favoured the escape of the fluid. This anatomist was also aware that the fluid was formed and contained in the sub-arachnoid cavity.
It is, however, to M. Majendie that we are chiefly indebted for our present knowledge of the physiological history of this fluid. Majen die's first researches were given to the public in his Journal de Physiolome for the year 1827, and he has lately collecte'd the results of his inquiries in a volume entitled " Recherches Physiologiques et Cliniques sur le Liquide Cephalo-rachidien," and published in 1842.
The cerchro-spinal fluid is found wherever pia mater exists in connexion with brain or spinal cord, whether on the surface of these organs, or in the ventricles of the former. It serves to fill up various inequalities in the cra nial or spinal walls, and it accumulates in greatest quantity in those situations where the sub-arachnoid space affords the greatest ca pacity.
Majendie describes four situations at which this fluid accumulates in greater quantity than at other places on the surface of the brain. The most considerable of these, which he desig nates the posterior court's:, is situated below and behind the cerebellum; it corresponds to the posterior surface of the medulla oblongata, and is covered behind by the layer of arachnoid which extends between the medulla and the cere bellum. (Vid.supr. p.638.) It is here that, ac cording to Majendie, a communication takes place between the fluid on the exterior and that in the ventricles, at a point corresponding to the inferior extremity of the fourth ventricle. A
second, or inferior conflux is found immedi ately in front of the pans Varolii ; it is situated betvveen the crura cerebri, and contains the basilar artery. It is, in fact, only the posterior part of what Majendie designates the anterior conflux, which extends forwards to the com missure of the optic nerves, occupying the central depression between the middle lobes of opposite sides, and bathing in its fluid the commissure, the tuber cinereum, the infundi bulum, and the trunks of the anterior cerebral arteries. It communicates with the posterior fissure beneath the crura cerebelli. The posi tion and the extent of this conflux is indicated by the sepamtion of the visceral layer of the arachnoid membrane over the central part of the base of the brain. Doubtless the accumu lation of thud around so many parts of impor tant function and delicate structure, is a va luable safeguard to them against the communi cation of shocks from the walls of the cra nium. A fourth conflux is called superior; it is situated behind and a little below the level of the corpus callosum, behind the pineal gland, and above the tubercula quadrigemina. It communicates around the crum cerebri with the anterior conflux, and with the posterior conflux by the fissures which sepamte the superior ver miform process from the hemispheres of the cerebellum. The fluid contained in it bathes the pineal gland, the tubercula quadrigemina, the superior vermiform process, and the venee Galeni as they empty themselves into the strait sinus.
As the fluid is in contact with pia mater, it is plain that it must surround and support the roots of all the nerves which proceed from both the brain and spinal cord, and that all the bloodvessels which penetrate or emerge from those organs, or which ramify in the pia mater, must also be bathed by it. The fluid surrounds the nerves as they emerge from the cranium or spine, and maintains contact between the layers of arachnoid membrane which compose the sheaths that accompany them in their passage outwards. Majendie states that this fluid accompanies the roots of the fifth pair of nerves as far as the Gasserian ganglion, and that it bathes and mingles with the fibres of the ganglion itself, as well as of the three nerves which originate from it. This, however, I think extremely doubtful.