THE BRAIN AND ITS MEMBRANES. 705 



mater internally and the dura mater externally ; from this latter membrane it is 

 separated by a space, the subdural space. 



It invests the brain loosely, being separated from direct contact with the 

 cerebral substance by the pia mater, and a quantity of loose areolar tissue, the 

 ntbaracknoidean. 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 toward the central part ; it covers the anterior lobes, and 

 extends across between the two temporo-sphenoidal lobes, so as to leave a consid- 

 erable interval between it and the brain, the anterior subarachnoidean space; it 

 is in contact with 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 subarachnoidean space. 

 These two spaces communicate together across the crura cerebelli. 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. 



The subarachnoid 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 temporo-sphenoidal 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 subarachnoid 

 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 subdural space also contains fluid; this is, however, small in quantity 

 compared with the cerebro-spinal fluid. 



Structure. The arachnoid consists of bundles of white fibrous and elastic 

 tissue intimately blended together. Its outer surface is covered with a laver of 

 emlothelium. From its inner surface are given off a number of bundles of fine 

 connective tissue, which form a sponge-like trabecular network in the subarachnoid 

 space, in the interstices of which the cerebro-spinal fluid is contained. Vessels of 

 considerable size, but few in number, and, according to Bochdalek, a rich plexus of 

 nerves derived from the motor division of the fifth, the facial, and the spinal 

 accessory nerves, are found in the arachnoid. 



The cerebro-spinal fluid fills up the subarachnoid space. 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, 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 concus- 

 sions communicated from without. 



The 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 blood-vessels, 

 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. It represents only the inner layer of the pia mater of the cord. 

 Upon the surfaces of the hemispheres, where it covers the gray matter of the 

 convolutions, it is very vascular, and gives off 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 anterior and posterior 

 perforated spaces, 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 



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