The cranial nerve-roots are designated by Roman numerals.
]
Its =floor= (Fossa rhomboidea) is formed by the =medulla= and =pons= and
is marked by three longitudinal furrows which converge behind. It is
widest and deepest a little in front of its middle. The posterior part
narrows to a point at the opening of the central canal, and on account
of its appearance in man it has been termed the calamus scriptorius. The
=median sulcus= (Sulcus medianus) extends the entire length of the floor
and is deepest toward the ends. The =limiting sulci= (Sulci limitantes)
begin on either side of the opening of the central canal and extend
forward as the lateral limits of the rhomboid fossa. Just beyond the
middle of the fossa they expand into a shallow depression, the =anterior
fovea= (Fovea nasalis). On either side of the median sulcus and margined
by the limiting sulcus is a slightly rounded column, the =eminentia
medialis=. Opposite the fovea this presents an elongated prominence, the
=colliculus facialis=, so named because it overlies the bend formed by
the fibers of origin of the facial nerve. External to the limiting
sulcus is a long fusiform elevation, the =area acustica=, from which a
band of fibers (Striæ acusticæ) winds over the anterior end of the
restiform body to the superficial origin of the cochlear nerve.
The =lateral wall= is formed by the restiform body and the anterior
peduncle of the cerebellum.
The =roof= is formed in its middle part by the vermis of the cerebellum,
covered by the epithelium before mentioned. There is commonly a dorsal
recess (Recessus tecti s. fastigium) between the extremities of the
vermis. The anterior part of the roof is formed by a thin lamina of
white substance, the =anterior medullary velum= (or valve of Vieussens),
which extends backward from the corpora quadrigemina, and is attached on
either side to the anterior peduncles of the cerebellum. Its anterior
part is relatively thick and contains the decussation of the fibers of
the trochlear nerves. Posteriorly it blends with the white matter of the
cerebellum. A thin lamina of white matter, the =posterior medullary
velum=, backed by pia mater, completes the roof posteriorly. After
removal of the cerebellum the line of attachment (Tænia ventriculi
quarti) to the medulla is seen; it begins centrally over the opening of
the central canal, runs forward on the inner face of the restiform body,
and turns outward behind the brachium pontis. The thick part which
stretches over the posterior angle of the ventricle is termed the
=obex=. The posterior part of the ventricle forms three recesses, of
which two are lateral and the third median and posterior. The =lateral
recesses= communicate with the subarachnoid space by distinct openings
(Aperturæ laterales). The layer of pia which strengthens the roof here
is named the =tela chorioidea= of the fourth ventricle. It is triangular
in outline and closely adherent to the velum. It forms three fringed
masses which contain vascular convolutions and are designated the
=median= and =lateral chorioid plexuses= of the fourth ventricle. They
appear to lie within the ventricle, but are really excluded from the
cavity by the epithelial lining, which they invaginate.
[Illustration: