410 CEREBRUM. 



This membrane differs very strikingly in its structure in different 

 parts of the cerebro-spinal axis. Thus, on the surface of the cerebrum, 

 in contact with the soft grey matter of the brain, it is excessively 

 vascular, forming remarkable loops of anastomoses in the interspaces 

 of the convolutions, and distributing multitudes of minute straight ves- 

 sels to the grey substance. In the substantia perforata, again, and 

 locus perforatus, it gives off tufts of small arteries, which pierce the 

 white matter to reach the grey substance in the interior. But upon 

 the crura cerebri, pons Varolii, and spinal cord, its vascular character 

 seems almost lost. It has become a dense fibrous membrane, difficult to 

 tear off, and forming the proper sheath of the spinal cord. 



The pia mater is the nutrient membrane of the brain, and derives 

 its blood from the internal carotid and vertebral arteries. 



Its Nerves are the minute filaments of the sympathetic, which ac- 

 company the branches of the arteries. 



CEREBRUM. 



The Cerebrum is divided superiorly into two hemispheres by the 

 great longitudinal fissure, which lodges the falx cerebri, and marks the 

 original development of the brain by two symmetrical halves. 



Each hemisphere, upon its under surface, admits of a division into 

 three lobes, anterior, middle, and posterior. The anterior lobe rests 

 upon the roof of the orbit, and is separated from the middle by the 

 fissure of Sylvius.* The middle lobe is received into the middle 

 fossa of the base of the skull, and is separated from the posterior by a 

 slight impression produced by the ridge of the petrous bone. The pos- 

 terior lobe is supported by the tentorium. 



If the upper part of one hemisphere, at about one-third from its 

 summit, be removed with a scalpel, a centre of white substance will be 

 observed, surrounded by a narrow border of grey, which follows the 

 line of the sulci and convolutions, and presents a zigzag form. This 

 section from exhibiting the largest surface of medullary substance de- 

 monstrable in a single hemisphere is called centrum ovale minus; it is 

 spotted by numerous small red points (puncta vasculosa) which are 

 produced by the escape of blood from the cut ends of minute arteries 

 and veins. 



Now separate carefully the two hemispheres of the cerebram, and a 

 broad band of white substance (corpus callosum) will be seen to con- 

 nect them ; it will be seen also that the surface of the hemisphere 

 where it comes into contact with the corpus callosum is bounded by a 

 large convolution (gyrus fornicatus) which lies horizontally on that 

 body, and may be traced forwards and backwards to the base of the 



* James Dubois, a celebrated professor of anatomy in Paris, where he suc- 

 ceeded Vidius in 1550, although known much earlier by his works and dis- 

 coveries, but particularly by his violence in the defence of Galen. His name 

 was Latinised to Jacobus Sylvius. 



