The parietal layer covers the inner surface of the dura mater, to which it is very intimately adherent, and gives this membrane the smooth and polished surface which it presents; it is also reflected over those processes which separate the hemispheres of the brain and cerebellum.

  The visceral layer invests the brain more loosely, being separated from direct contact with the cerebral matter by the pia mater, and a quantity of loose areolar tissue, the sub-arachnoidean. On the upper surface of the cerebrum the arachnoid is thin and transparent, and may be easily demonstrated by injecting a stream of air beneath it by means of a blowpipe; it passes over the convolutions without dipping down into the sulci between them. At the base of the brain, the arachnoid is thicker, and slightly opaque towards the central part; it covers the anterior lobes, is extended across between the two middle lobes, so as to leave a considerable interval between it and the brain, the anterior sub-arachnoidean space; it is closely adherent to the pons and under surface of the cerebellum, but between the hemispheres of the cerebellum and the medulla oblongata another considerable interval is left between it and the brain, called the posterior sub-arachnoidean space. These two spaces communicate together across the crura cerebri. The arachnoid membrane surrounds the nerves which arise from the brain, and encloses them in loose sheaths as far as their point of exit from the skull, wkere it becomes continuous with the parietal layer.

  The Sub-arachnoid Space is the interval left between the arachnoid and pia mater: this space is narrow on the surface of the hemispheres, but at the base of the brain a wide interval is left between the two middle lobes, and behind, between the hemispheres of the cerebellum and the medulla oblongata. This space is the seat of an abundant serous secretion, the cerebro-spinal fluid, which fills up the interval between the arachnoid, and pia mater. The sub-arachnoid space usually communicates with the general ventricular cavity of the brain, by means of an opening in the inferior boundary of the fourth ventricle.

  The sac of the arachnoid also contains serous fluid; this is, however, small in quantity compared with the cerebro-spinal fluid.

  Structure. The arachnoid consists of bundles of white fibrous and elastic tissues intimately blended together. The visceral portion is covered with a layer of scaly epithelium. It is almost destitute of vessels, and the existence of nerves in it has not been satisfactorily demonstrated.

  The Cerebro-spinal Fluid fills up the sub-arachnoid space, keeping the opposed surfaces of the arachnoid membrane in contact. It is a clear limpid fluid, having a saltish taste, and a slightly alkaline reaction. According to Lassaigne, it consists of 98'5 parts of water, the remaining 1*5 per cent, being solid matters, animal and saline. It varies in quantity from two to ten ounces, being most abundant in old persons, and is quickly reproduced. Its chief use is probably to afford mechanical protection to the nervous centres, and to prevent the effects of concussion communicated from without.