PIA MATER THE BRAIN. 579 



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 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 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 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 subarachnoid 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, 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 lamina?, and is pro- 

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

 of the fourth ventricle. 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 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 deli- 

 cate, and the vessels from its inner surface are shorter. Upon the crura cerebri 

 and pons Varolii its characters are altogether changed ; it here presents a dense 

 fibrous structure, marked only by slight traces of vascularity. 



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

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

 third, sixth, seventh, eighth, and spinal accessory. They accompany the branches 

 of the arteries. 



-THE BRAIN. - 



The Brain (encephalon] is that .portion of the cerebro-spinal axis that is con- 

 tained in the cranial cavity. It is divided into four principal parts: viz., the 

 cerebrum, the cerebellum, the pons Varolii, and medulla oblongata. 



The cerebrum forms the largest portion of the encephalon, and occupies a 

 considerable part of the "cavity of the cranium, resting in the anterior and middle 

 fossa? of the base of the skull, and separated posteriorly from the cerebellum by 

 the tentoriurn cerebelli. About the middle of its under surface is a narrow 

 constricted portion, part of which, the crura cerebri, is continued onwards into 

 the pons Varolii below, and through it to the medulla oblongata and spinal cord; 

 whilst another portion, the crura cerebelli, passes down into the cerebellum. 



