THE CEREBELLUM. 



235 



by two transverse fissures into (a) anterior, (b) middle and (c) 

 posterior lobes. The transverse fissures are named the fissura 

 prima and fissura secunda. These divisions are shown in a 

 median sagittal section of the marsupial cerebellum in Fig. 118 

 and in a diagrammatic dorsal view of the higher mammalian 

 brain in Fig. 119. The anterior and posterior lobes do not expand 

 laterally but are formed from the region adjacent to the mid- 

 dorsal line. They become divided by later fissures into subsidiary 

 lobules which are of no importance in the present connection. The 

 anterior lobe includes at its cephalic border the lingula which is 

 connected by means of the valve o) Vieussens with the corpora 

 quadrigemina. This region therefore corresponds in position to 

 the velum medullare anterius of the selachian brain and the valvula 

 cerebelli of the brain of ganoids and teleosts. The posterior lobe 

 includes the two transverse folds known as the uvula and nodulus, 

 to the latter of which is attached the membranous roof of the fourth 

 ventricle. The nodulus therefore corresponds to the meso-caudal 

 border of the cerebellum to which the choroid plexus is attached 

 in all vertebrates. (Compare Figs. 118 B and 120.) The middle 

 lobe in the embryo extends laterally and connects with the extreme 

 lateral portions which form the floccular lobe. The middle lobe 

 is divided into a median portion or vermis and lateral lobes, each 

 of which is further divided by subsidiary transverse fissures. 

 The anterior lobe corresponds to the larger part of the superior 

 vermis of anatomists, while the inferior vermis includes the pos- 

 terior lobe and most of the median portion of the middle lobe. 



This brief survey of the surface characters of the cerebellum 

 gains in significance when the structure and arrangement of its 

 deeper parts are considered. Throughout the whole cerebellum 

 except the anterior lobe it consists of the following layers from 

 without inward, (i) Molecular layer, consisting of (a) cells, 

 (b) non-myelinated fibers derived from the granule cells, and (c) 

 the dendrites of Purkinje cells. (2) Layer of Purkinje cell- 

 bodies. (3) Granular layer consisting chiefly of granule cells. (4) 

 Layer of myelinated fibers. This layer is very voluminous and 

 its subdivision to the various lobes gives rise to the well known 

 arbor vitae. In the region of the anterior lobe the structure is 



