THE CEREBELLUM 



805 



separated by narrow but relatively deep sulci. Unlike the spinal cord and 

 medulla, in which the grey substance is centrally placed and surrounded by a 

 mantle of white substance, the surface of the cerebellum is itself a cortex of grey 

 substance, the cortical substance [substantia corticalis], enclosing a core of white 

 substance, the medullarij body [corpus medullare]. However, within this central 

 core of white substance are situated definite grey masses, the nuclei of the 

 c^e bellum. 



The gross divisions of the cerebellum are three: the two larger lateral portions, 

 the hemispheres, and between these the smaller central portion, the vermis. 

 The demarcation between these gross divisions is not very evident from the dorsal 

 surface, because the hemispheres in their extraordinary development in man 

 encroach upon the vermis, and, being pressed under the overlapping occipital 

 ends of the cerebral hemispheres, they become partially fused upon the vermis 



Fig. 633. — Section of Head Passing Through the Mastoid Processes of the Temporal 

 Bones and Behind the Medulla Oblongata. Showing the Position op the Cere- 

 bellum. 

 (From a mounted specimen in the Anatomical Department of Trinity College, Dublin.) 



Superior sagittal 

 sinus 



Falx cerebri 



Corpus callosum — 



Chorioid plexus — 



Veins of Galen 



Tentorium 

 cerebelli 



Transverse sinus 



Dentate nucleus 



Caudate nucleus 



l-^-T — Lateral ventricle 



Superior petrosal 



sinus 

 Mastoid antrum 



TraBSverse sinus 



Mastoid process 



along the dorsal mid-line. Though differentiated simultaneously with the cere- 

 bellar hemispheres in the human foetus, in most of the mammalia the vermis is 

 the largest and most evident of the parts, and it is practically the only part which 

 exists in the fishes, reptiles, and birds. In man, owing to the fact that the vermis 

 does not keep pace in development with the hemispheres, there results a very 

 decided notch between the two hemispheres along the line of the entire ventral and 

 inferior aspect of the cerebellum, the floor of this notch being the surface of the 

 vermis. The inferior portion of the notch is the posterior cerebellar notch 

 (incisura marsupialis) ; its prolongation above is wider than below, and is termed 

 the superior cerebellar notch. It is occupied by a fold of the dura mater, the 

 fak cerebelli. With the variations in contour of the cerebellum, certain of its 

 sulci are broader and deeper, and merit the name fissures. These are more or less 

 definitely placed, and subdivide the hemispheres into lobes and the vermis (the 

 median lobe) into lobules. 



Superior surface. — The superior surface is bounded from the inferior surface 

 by the horizontal fissure (fig. 635) which extends ventrolaterally, to the entrance of 

 the brachium of the pons. Between this and the extreme anterior border of the 

 dorsal surface are two other fissures, the posterior and anterior semilunar fissures. 

 These, like the horizontal fissure, may be traced, with slight interruptions, across 

 the mid-line, and consequently mark off not only the two hemispheres but also the 

 vermis into corresponding divisions. 



