THE CEREBELLUM 



1247 



received largely vestibular and spinal afferents 

 whereas the neocerebellum received afferents acti- 

 vated from the cerebral cortex. The physiological 

 justification for the distinction between the paleo- 

 and neo-portions of the cerebellum was first offered 

 by Bremer (36). Further studies of comparative and 

 developmental anatomy have led Herrick (157) and 

 Larsell (180) to reinforce this distinction. More re- 

 cent anatomical studies (169), and the evidence 

 which will be reviewed, indicate that this partition 

 of functional relationships is too rigid and that 

 cerebrocerebral relations must be mediated in part 

 by the more anterior portions of the paleocerebellum. 



Undoubtedly the most universally applicable and 

 useful description of the gross morphology of the 

 cerebellum is that which has grown out of Larsell's 

 comparative and embryological studies (180). The 

 basic subdivision into a flocculonodular lobe sepa- 

 rated from a corpus cerebelli by a posterolateral 

 fissure is applicable to all species studied. Larsell 

 further subdivides the corpus cerebelli into an an- 

 terior and a posterior lobe. The further subdivision of 

 the corpus cerebelli into lobules has been accom- 

 plished in the past through the use of terms which 

 were not universally applicable. In what is to follow, 

 an attempt is made to use current terminology along 

 with the numerical designation recommended by 

 Larsell (181). Figure i is an attempt to represent the 

 morphology of the cerebellum. The additional divi- 

 sion of the cerebellum into sagittal divisions con- 

 sisting of a vermis, and an intermediate and lateral 

 portion of each hemisphere seems indicated by both 

 anatomical and physiological evidence which will be 

 discussed later. 



The functional importance of this gross morpho- 

 logical subdivision of the cerebellum resides in two 

 facts. The flocculonodular lobe is the only portion of 

 the cerebellum which has direct two-way connec- 

 tions with the vestibular system. The phylogenetic 

 development of the cerebral cortex is coupled with 

 the phylogenetic development of the lateral portions 

 of the cerebellar hemispheres. 



Cortex 



The outstanding histological characteristic of the 

 large expanse of highly infolded cerebellar cortex is 

 its uniformity in all parts of the cerebellum. This 

 cortex is divisible histologically into three layers: a 

 superficial molecular layer, at the bottom of which 

 is found the row of Purkinje cells; and a deep granu- 

 lar layer overlying the white matter. Neurons are 



relatively sparsely scattered in the superficial portion 

 of the molecular layer, the bulk of the volume being 

 comprised of the tremendous dendritic expansions 

 of the Purkinje cells enveloped with climbing fibers, 

 the ascending and bifurcating axons of the granule 

 cells of the third layer, and the axons and dendrites 

 of the star cells and basket cells of the molecular 

 layer. At the base of the molecular layer the row of 

 Purkinje cells is composed of the large globose somata 

 of these cells arranged almost in contact with each 

 other, separated by the basketwork of axonal termi- 

 nations derived from the basket cells of the molec- 

 ular layer. The granular layer is composed chiefly 

 of the densely packed bodies of the granule cells, 

 their complex dendritic expansions which intertwine 

 with the terminals of the incoming mossy fibers, 

 Golgi type II cells and the incoming and outgoing 

 fibers of passage. 



The impulses to the cerebellar cortex are delivered 

 over the mossy fibers from such diverse sources as 

 the spinal cord, the cerebral cortex via the pontine 

 nuclei, the reticular nuclei of the tegmentum and 

 medulla, and the \estibular system (cf. 169). The 

 origin of the impulses delivered to the cortex by the 

 climbing fibers still remains in doubt, the suggestion 

 that they originate as recurrent collaterals of axons of 

 intracerebellar nuclei (63) not being acceptable to 

 all (292). 



Efferent impulses from the cerebellar cortex are 

 discharged entirely over the axons of the Purkinje 

 cells, there being no other known axon which leaves 

 the cortex to enter the white matter. 



Nuclei 



The cell bodies of the cerebellar nuclei constitute 

 the end station of the vast majority of Purkinje cell 

 axons. In step with, and as a reflection of, the phy- 

 letic development of the cerebellum, the cerebellar 

 nuclei undergo variations from species to species. In 

 the mammal, the medial nuclear group, the fastigial 

 nucleus, is the oldest from the phylogenetic point of 

 view. Its extracerebellar input is dominated in- the 

 vestibular inflow and its output is principally directed 

 to bulbar areas. The intermediate group is homolo- 

 gous with the interpositus in lower mammals and 

 with the nuclei globosus and emboliformis in pri- 

 mates. The lateral group is homologous with the 

 dentate nucleus of the primate. 



The corticonuclear relations of the cerebellar 

 nuclei have been satisfactorily clarified for the cat 

 (167), rabbit and monkey (168) by Jansen & Brodal. 



