PIA MATER. 509 



The visceral layer invests the brain more loosely, being separated from direct 

 contact with the cerebral substance 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 hemi- 

 spheres 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 mem- 

 brane surrounds the nerves which arise from the brain, and incloses them in loose 

 sheaths as far as their point of exit from the skull, where it becomes continuous 

 with the parietal layer. 



The sub-arachnoid space is the interval 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. Its outer surface is covered with a layer of scaly 

 epithelium. It is 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 con- 

 sists of 98.5 parts of water, the remaining 1.5 per cent, being solid matters, animal 

 and saline. It varies in quantity, 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 concussions communicated from 

 without. 



PIA MATER. 



The Pia Mater is a vascular membrane, and derives its blood from the internal 

 carotid and vertebral arteries. It consists of a minute plexus of bloodvessels, 

 held together by an extremely fine areolar tissue. It invests the entire surface 

 of the brain, dipping down between the convolutions and laminae, and is prolonged 

 into the interior, forming the velum interpositum and choroid plexuses of the 

 fourth ventricle. Upon the surface of the hemispheres, where it covers the gray 

 matter of the convolutions, it is very vascular, and gives oft' from its inner surface 

 a multitude of minute vessels, which extend perpendicularly for some distance 

 into the cerebral substance. At the base of the brain, in the situation of the 

 substantia perforata and locus perforatus, a number of long straight vessels are 

 given off, which pass through the white matter to reach the gray substance in 

 the interior. On the cerebellum, the membrane is more delicate, and the vessels 

 from its inner surface are shorter. Upon the crura cerebri and pons Yarolii, its 

 characters are altogether changed; it here presents a dense fibrous structure, 

 marked only by slight traces of vascular ity. 



According to Fohmann and Arnold, this membrane contains numerous lym- 

 phatic vessels. Its nerves are derived from the sympathetic, and also from the 



