terminate in unbranched ends. The large granule
cells are multipolar cells, many of the branchings penetrating well
into the molecular layer. The neuraxon process turns into the opposite
direction and forms a richly branching system through the entire
thickness of the granular layer. There is also an abundant plexus of
fine medullated fibres within the granule layer.

The fibres of the white central matter are partly centrifugal, the
neuraxons of the cells of Purkinje, and partly centripetal. The
position of the cells of these latter fibres is not known. The fibres
give rise to an abundant plexus of fibrils in the granular layer, and
many reaching into the molecular layer ramify there, especially in the
immediate neighbourhood of the dendrites of Purkinje's cells. From the
appearance of their plexus of fibrils these are sometimes called _moss
fibres_.

The _Fourth Ventricle_ is the dilated upper end of the central canal
of the medulla oblongata. Its shape is like an heraldic lozenge. Its
floor is formed by the grey matter of the posterior surfaces of the
medulla oblongata and pons, already described (see figs. 3 and 6); its
roof partly by the inferior vermis of the cerebellum, the _nodule_ of
which projects into its cavity, and partly by a thin layer, called
_valve of Vieussens_, or superior _medullary velum_; its lower lateral
boundaries by the divergent clavae and restiform bodies; its upper
lateral boundaries by the superior peduncles of the cerebellum. The
_inferior medullary velum_, a reflection of the pia mater and
epithelium from the back of the medulla to the inferior vermis, closes
it in below. Above, it communicates with the _aqueduct of Sylvius_,
which is tunnelled below the substance of the corpora quadrigemina.
Along the centre of the floor is the median furrow, which terminates
below in a pen-shaped form, the so-called _calamus scriptorius._
Situated on its floor are the fasciculi teretes, striae acusticae, and
deposits of grey matter described in connexion with the medulla
oblongata. Its epithelial lining is continuous with that of the
central canal.

_The Cerebrum._

The _Cerebrum_ or GREAT BRAIN lies above the plane of the tentorium,
and forms much the largest division of the encephalon. It is customary
in human anatomy to include under the name of cerebrum, not only the
convolutions, the corpora striata, and the optic thal