INTERNAL PARTS. 537 



the Sylvian fissure, where it disappears. It has been traced by different 

 observers to the island of Reil, the optic thalamus (Valentin), and to a 

 nucleus ia tlie substance of the temporo-sphenoidal lobe, in front of 

 the anterior extremity of the hippocampus. (Eolaudo, Foville, Luys.) 

 The middk, or grey root (tuber olfactorium), is of pyramidal shape, 

 and consists of grey substance on the snrface, prolonged from the adja- 

 cent part of the anterior lobe and anterior perforated space. Within 

 it are some white fibres, which have been said by some to join the inner 

 root, by others to be connected with the corpus striatum. The i/nier 

 root consists of white fibres, which pass to the inner and posterior part 

 of the anterior lobe, and are believed to be connected with the extremity 

 of the gyrus fornicatus, or to pass over to the opposite hemisphere. 



When the entire encephalon is viewed from below, the back part of 

 the under surface of the cerebrum is concealed by the cerebellum and 

 the pons Varolii. If, however, these parts be removed, it will be seen 

 that the two Iiemispheres of the cerebrum are separated behind as they 

 are in ft-ont, by the descent of the great longitudinal fissure between 

 them, behind the posterior extremity of the corpus callosum. 



INTERNAL PAPoa?S OF THE CEREBRUM. 



The anatomy of the interior of tlie cerebrum is most conveniently 

 studied by removing successive portions of the hemispheres by hori- 

 zontal sections, beginning from above. 



The first horizontal section to be made about half an inch above the 

 corpus callosum, displays the internal white matter of each hemisjjhere, 

 speckled with red spots where its blood-vessels have been divided, and 

 surrounded on all sides by the grey matter which is seen to follow 

 closely the convoluted surface, and to be of nearly equal thickness at 

 all points. This white central mass in each hemisphere was named by 

 Vicq-d'Azyr centrum ovale minus. On separating the remaining por- 

 tions of the two hemispheres from each other, two sulci are seen to exist 

 between the corpus callosunr and the gyri immediately in contact with 

 it, viz., the gyrus fornicatus of each side. 



Another section being made at the level of the corpus callosum, the 

 white substance of that part is seen to be continuous with the internal 

 medullary matter of both hemispheres : and the large white medullary 

 mass thus displayed, surrounded by the border of cortical substance, con- 

 stitutes what is generally described as the centrum ovale of Vieussens. 



The coi'piis callosiim or great commissure (tiabs cerebri) 

 (fig. 377) connects the centres of the two hemispheres. It is a white 

 structure, with a length not quite half that of the brain, and ap- 

 proaches nearer to the front than the back of the hemispheres. It is 

 about an inch in width behind, and somewhat narrower in fi-ont. Its 

 thickness is greater at the ends than in the middle, and is greatest 

 behind, where it is nearly half an inch. It is arclied from before 

 backwards. Its upper surfixce is distinctly marked by transverse fur- 

 rows, which indicate the direction of the greater number of its fibres. 

 It is also marked in the middle by a longitudinal line, the raphe, 

 which is bounded laterally by two white tracts, placed close to each 

 other, named strice longituclinales, or nerves of Lancisi (fig. 377, 3). On 

 each side, near the margin, are seen other longitudinal lines (strife 

 longitudinales laterales) occasioned by a few scanty white fibres. 



