90 J. B. JOHNSTON, 
layer of the lateral lobe. Regarding the lateral lobes as the two 
pillars of an arch and following them mesially, we find that the 
granular layer becomes enormously thickened dorso-ventrally and 
hangs down into the cavity of the arch, while the molecular layer 
dips down between the granular layer of the two sides after the 
manner of the keystone of an arch. This great mass I shall call 
the body of the cerebellum. The mesial part of the acusticum and 
the nucleus of the ascending V tract become continuous with the 
lateral part of the body. The cerebellar crest, when it joins the 
molecular layer is not lost as a distinct bundle, but continues me- 
sially and then caudo-mesially along the border of the lateral lobes 
at their junction with the plexus choroideus, and finally forms the 
most caudal and dorsal portion of the molecular layer in the middle 
line. Here these bundles do not enter what I have termed the 
keystone of the arch, but pass across the middle line, the fibres of 
the two sides decussating, and enter the lateral portion of the body 
of the cerebellum. The fibres of the cerebellar crest have their 
cells of origin here, as will be described below. The body of the 
cerebellum is continuous cephalad with the valvula, but the median 
molecular and lateral granular layers of the body become much 
less distinct in the valvula. The valvula is penetrated from the 
dorsal surface by a deep but narrow transverse cleft or fissure 
filled with connective tissue. This cleft marks externally the caudal 
boundary of the tectum. Its cephalic limb is the velum medullare 
anterius. The connection of the body of the cerebellum with the 
lateral walls of the mid brain is short in cephalo-caudal extent. 
The cephalic part of this connecting bridge may be considered as 
lying within the limits of the mid brain, and is occupied by bundles 
of fibres entering the cerebellum from the tectum and the lobi in- 
feriores. The foregoing description of the cerebellum is based upon 
the study of its minute structure and is prefaced here in order to 
make the description of the minute structure more clear. 
Although the lateral lobes of the cerebellum present the more 
“typical” arrangement of the layers and PURKINJE cells, the body 
and valvula are better impregnated in my preparations and I shall 
begin with a detailed description of the nerve elements in those 
parts. To describe the body first will be more convenient also be- 
cause most of the fibre tracts coming to the cerebellum enter by 
way of the body. 
