372 CEREBRUM. 



distributing multitudes of minute straight vessels 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 spina 

 cord. 



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

 blood from the internal carotid and vertebral arteries. 



Its Nerves are the minute filaments of the sympathetic, which accom 

 pany the branches of the arteries. 



CEREBRUM. 



The Cerebrum presents on its surface a number of slightly convex ele- 

 vations, the convolutions, (gyri) which are separated from each other by 

 sulci of various depth.* It 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, f 

 The middle lobe is received into the middle fossaB 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 posterior 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 exhibit- 

 ing the largest surface of medullary substance demonstrable 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 cerebrum, and a 

 broad band of white substance (corpus caJlosum) will be seen to connect 

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

 in 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 brain, terminating by each ex- 

 tremity at the fissure of Sylvius. The sulcus between this convolution and 

 the corpus callosum has been termed, very improperly, the lt ventricle of 

 the corpus callosum," and some longitudinal fibres (striaB longitudinales 

 laterales), which are brought into view when the convolution is raised, 

 were called by Reil the " covered band." If, now, the upper part of each 

 hemisphere be removed to a level with the corpus callosum, a large ex- 

 panse of medullary matter, surrounded by a zigzag line of grey substance 

 corresponding with the convolutions and sulci of the two hemispheres, will 

 be seen ; this is the centrum ovale majus of Vieussens. 



*In estimating the surface of the brain, which, according to Baillarger, averages in 

 lound numbers, 670 square inches, these convolutions and the laminae of the cerebellum 

 are supposed to be unfolded. 



f James Dubois, a celebrated professor of anatomy in Paris, where he succeeded Vi 

 dius in 1550, although known much earlier by his works and discoveries, but particu 

 latly by his violence in the defence of Galen. His name was latinised to Jacobus Sylvius. 



