THE CEREBELLUM. 467 
The posterior longitudinal bundle, as it is traced upwards through the tegmental 
part of the pons, maintains the same position throughout, and as it ascends it 
becomes more clearly mapped out as a definite and distinct tract. It lies close 
to the mesial raphe and immediately subjacent to the gray matter of the floor of 
the fourth ventricle. 
The fillet as it ascends through the tegmental part of the pons undergoes 
striking changes in shape. In the lower portion of the pons its fibres, which in 
the medulla are spread out along the side of the median raphe, are collected 
together in the form of a loose Dundle, which occupies a wide field, somewhat 
triangular in shape, on either side of the median raphe and immediately behind 
Upper end of ventricle IV. 
Trochlear nerve 
Mesencephalic root of fifth nerve 
Gowers’ tract ass 
Inferior quadri- 
geminal body 
Decussation 
; of trochlear 
nerves 
Upper end of 
ventricle IV. 
~~ 
Posterior longi- 
tudinal bundle —~_ Lo Veg 
Superior cere- 
bellar peduncle _ 
| Superior cere- 
bellar peduncle 
Posterior longi- 
¥ tudinal bundle 
Lateral fillet +3hy), Lateral fillet 
Formatio 
reticularis 
Mesial fillet Mesial fillet 
Fic. 347.—Two SEcTIONS THROUGH THE TEGMENTUM OF THE PONS AT ITS UPPER PART, CLOSE TO 
THE MESENCEPHALON. 
A is at a slightly lower level than B. 
the ventral portion of the pons. As it proceeds up, the fibres spread out laterally 
until a compact ribbon-like layer is formed in the interval between the tegmental 
and ventral portions of the pons. This constitutes what is termed the mesial 
fillet (Figs. 546 and 547). 
Above the level of the trigeminal nuclei another flattened layer of fibres comes 
into view to the outer side of the mesial fillet. To this the name of lateral fillet is 
given. These fibres spread outwards and backwards, and finally take up a position 
on the outer surface of the superior cerebellar peduncle. In the angle between 
the mesial and lateral fillets a little knot of compact gray matter, termed the lateral 
fillet nucleus, comes into view (Fig. 546). This appears to be in more or less direct 
continuity with the superior olivary nucleus. Many of the fibres of the lateral 
fillet take origin in this nucleus. Bruce has called attention to the continuity 
between the superior olive and the lateral fillet nucleus in man, and the writer 
can confirm his statement in so far as the orang brain is concerned. 
THE CEREBELLUM. 
The cerebellum lies behind the pons Varolii and the medulla oblongata, and 
below the hinder portions of the cerebral hemispheres, from which it is separated 
by the intervening partition of dura mater, termed the tentorium cerebelli. It is 
distinguished by “the numerous parallel and more or less curved sulei, which 
traverse its surface and give it a foliated or laminated appearance. It is composed 
of a cortex of gray matter (substantia corticalis) spread over its surface, with white 
matter in the interior, forming a central core (corpus medullare). 
In the cerebellum we recognise a median portion termed the vermis, and two 
much larger lateral portions, called the lateral hemispheres (hemispheeria gerebelli): 
The demarcation between these main subdivisions of the organ is not very evident 
from every point of view. In front, and also behind, there is a marked deficiency 
