558 THE CEREBEUM. 



2. The fibres of the ierjmentiun, having entered the thalamus, pass on in two 

 ■ways — no part of tliem, however, joining- the radiating- peduncular fibres. 



a. One set pass upwards through the thalamus and corpus striatum, above 

 ■which they then turn inwards, and, joining with those of the opposite side, 

 form the transverse fibres of the corpus callosimi. The corpus callosum is there- 

 fore regarded as a commissure of the cerebral peduncles only — none of its cross 

 fibres spreading into the convolutions, as is generally believed. 



h. The second set of fibres of the teg-mentum, corresponding with the fasciculi 

 teretes and part the posterior pjTamids, run forwards near the middle line, along 

 the under side of the third ventricle and corpus striatum, through the grey 

 matter in front of the pons, to the anterior perforated space. The remaining 

 pai-t of the posterior pyi-amid forms the taenia semicircularis, which, passing 

 down in front of the anterior pillar of the fornix, also reaches the perforated 

 space. From this space more fibres are reflected upwards on the sides of the 

 corpus striatum to join the corpus callosiim. 



3. As dependencies of the posterior peduncular fibres, and connected with them 

 at the borders of the anterior perforated space, are : — 



a. Several sets of longitudinal arched fibres, which embrace, in a series of rings, 

 the radiating peduncular system. These are — the deep fibres of the ttenia semi- 

 circularis — a somewhat similar band beneath the outer part of the corpus striatiim 

 — the half of the fornix with the coii^us fimbriatum — the longitudinal fibres 

 placed on the upper and under surface of the corpus callosum, and those of the 

 septum lucidum ; and, lastly, two remarkable systems of longitudinal fibres — one 

 constituting the entire white substance of the gyi-us f ornicatus (fi-om end to end), 

 also of its accessory convolutiop,.s, and of the inner half of the marginal convo- 

 lution of the longitudinal fissure ; and the other, forming the white substance of 

 the convolutions of the island of Reil, and the adjoining half of the convolution 

 of the Syl-vian fissure. None of the parts just named receive fibres from the 

 radiating peduncular set. 



h. In connection with this system is a thin stratum of white fibres, lomid 

 upon the internal surface of the ventricles, and prolonged through the transverse 

 fissure into the reticulated white substance covering the lower end of the g.yrus 

 fornicatus ; whence it extends, as an exceedingly thin layer of medullary 

 matter, all over the coi-tical substance of the hemisphere. 



c. The anterior commissiu-e does not reach the convolutions, but radiates upon 

 the outer sides of the corpora striata and thalami. 



The Grey Matter of the cerebrum may be considered in three 

 categories, according as it is placed («) on the convoluted surface, {h) at 

 the base, and {c) in tlie interior of the cerebrum. 



(A.) ON THE CONVOLUTED SURFACE the grey matter forms a continuous 

 layer divided into two and in some regions into three strata, by interposed 

 thin layers of paler substance. In examining a section from without in- 

 wards, we meet with — 1. A thin coating of white matter situated on the 

 surface, which on a section appears as a faint white line, bounding the 

 grey surface externally (fig. 390, a). This superficial white layer is not 

 equally thick over all parts of the cortical substance, but becomes thicker 

 as it approaches the borders of the convoluted surface ; it is accordingly 

 less conspicuous on the lateral convex aspect of the hemispheres, and 

 more so on the convolutions situated in the longitudinal fissure which 

 approach the white surface of the corpus callosum, and on those of the 

 under surface of the brain. It is especially well marked on the temporo- 

 sphenoidal lobe, near the descending cornuof the lateral ventricle, where 

 the convoluted surface is bounded by the posterior pillar of the fornix, and 

 it has been there described under the name of the refkulated wliitc sub- 

 siance. 2. Immediately beneath the white layer just described, is found 

 a comparatively thick layer of grey or reddish grey matter, the colour 

 of which, as indeed of the grey substance generally, is deeper or lighter 



