aped _olivary body_. Behind the olivary body in the
lower half of the medulla are three tracts named from before backward
the _funiculus of Rolando_, the _funiculus cuneatus_ and the
_funiculus gracilis_ (see fig. 3). The two _funiculi graciles_ of
opposite sides are in contact in the mid dorsal line and have between
them the _postero median_ fissure. When the fourth ventricle is
reached they diverge to form the lower limit of that diamond-shaped
space and are slightly swollen to form the _clavae_. All these three
bundles appear to be continued up into the cerebellum as the restiform
bodies or inferior cerebellar peduncles, but really the continuity is
very slight, as the restiform bodies are formed from the direct
cerebellar tracts of the spinal cord joining with the superficial
arcuate fibres which curve back just below the olivary bodies. The
upper part of the fourth ventricle is bounded by the superior
cerebellar peduncles which meet just before the inferior quadrigeminal
bodies are reached. Stretching across between them is the superior
medullary velum or valve of Vieussens, forming the upper part of the
roof, while the inferior velum forms the lower part, and has an
opening called the _foramen_ of Majendie, through which the
sub-arachnoid space communicates with the ventricle. The floor (see
fig. 3) has two triangular depressions on each side of a median
furrow; these are the superior and inferior _fovea_, the significance
of which will be noticed in the development of the rhombencephalon.
Running horizontally across the middle of the floor are the _striae
acusticae_ which are continued into the auditory nerve. The floor of
the fourth ventricle is of special interest because a little way from
the surface are the deep origins of all the cranial nerves from the
fifth to the twelfth. (See NERVE, _cranial_). If a section is made
transversely through the medulla about the apex of the fourth
ventricle three important bundles of fibres are cut close to the mid
line on each side (see fig. 4). The most anterior is the pyramid or
motor tract, the decussation of which has been seen. Behind this is
the mesial fillet or sensory tract, which has also decussated a little
below the point of section, while farther back still is the posterior
longitudinal bundle which is coming up from the anterior basis bundle
of the cord. External to and behind the pyramid is the