THE CEREBRUM. 



299 



pigmented. Numerous strands of nerve-fibres subdivide the nucleus into 

 secondary areas, while some large transversely coursing bundles establish 

 a decussation with the roof-nucleus of the opposite side. Notwithstanding 



FIG. 346. Transverse section through fourth ventricle and surrounding parts of brain-stem and of 

 cerebellum and its internal nuclei; i, pyramidal tracts; 2, mesial fillet; 3, posterior longitudinal fascic- 

 ulus; 4, choroid plexus; 5, cerebellar folia; 6, medulla ; 7, fourth ventricle; 8, nucleus dentatus; 9, 

 inferior worm ; 10, nucleus fastigii or roof-nucleus ; n, nucleus globosus ; 12, nucleus emboliformis ; 13, 

 nucleus dentatus; 14, choroid plexus; 15, restiform body; 16, fibres of vagus nerve; 17, spinal root of 

 fifth nerve; 18, lateral recess ; 19, olivary nucleus. X 3%- (Preparation by Professor Spiller.) 



their small relative size, the dentate and roof-nuclei are important stations, 

 since from their cells arise the fibres composing the greater part of the 

 superior cerebellar peduncle. The dentate nuclei are of additional interest 

 as the destination of many axones from the Purkinje cells. 



THE CEREBRUM. 



The cerebrum the ' ' great brain ' ' as distinguished from the cerebellum 

 comprises, in a general way, the large hemispheres and the parts surround- 

 ing the third ventricle. With the exception of where the peduncles enter 

 and the portion of the mesial surface below the great bridge, the corpus callo- 

 sum, connecting the hemispheres, the cerebrum is everywhere invested with a 

 continuous sheet of gray matter, the cerebral cortex. On account of the 

 complex convolutions of the surface, the cortex is thrown into conspicuous 

 folds that mark the convolutions and the intervening fissures and enclose the 

 penetrating tracts of white matter prolonged from the general medullary sub- 

 stance. The cortical sheet varies in thickness not only in the same area, 

 being thicker over the summit than along the sides of the convolutions or at 

 the bottom of the fissures, but in different regions. Its average thickness is 

 about 3 mm. , but where it borders the upper end of the Rolandic fissure, 

 this increases to almost 5 mm. , whilst over the occipital poles the thickness 

 of the cortex is reduced to about 2 mm. The entire superficial extent of the 

 cortex has been estimated to be about 2000 sq. cm. , of which scarcely one 

 third is exposed surface, the remainder being sunken. According to Don- 

 aldson, the cortex contributes about one half of the weight of the brain. 



