Section passes through posterior part of third ventricle and is viewed
from behind. _1_, Longitudinal fissure; _2_, hippocampus; _2′_,
fimbria; _3_, septum pellucidum; _4_, lateral ventricle; _5_,
thalamus; _6_, habenula; _7_, third ventricle; _8_, cerebral
peduncle; _8′_, hypothalamus; _9_, mammillary body; _10_, pituitary
body; _11_, pyriform lobe; _12_, ventral end of hippocampus; _13_,
amygdaloid nucleus. Between the upper parts of the tæniæ thalami is
the chorioid plexus of the third ventricle, and above this are the
internal cerebral veins.
]
All the fibers in the chiasm are not derived from the optic nerves.
The posterior part contains fibers which pass from one tract to the
other and are connected with the internal geniculate bodies; this
bundle is called Gudden’s commissure (Commissura inferior). Above it
is Meynert’s commissure (Commissura superior), the fibers of which
enter the subthalamic body.
The =third ventricle= (Ventriculus tertius) is the narrow annular space
between the thalami. It communicates by means of the aqueduct with the
fourth ventricle behind, and in front it is continuous with the lateral
ventricle on each side through the interventricular foramen. Its floor
is formed by the structures of the interpeduncular space and to a small
extent by the tegmentum of the cerebral peduncles. The roof is formed in
the strict sense only by the ependyma, above which is a fold of pia
mater, termed the =tela chorioidea= of the third ventricle or =velum
interpositum=. The roof is invaginated by two delicate =chorioid
plexuses= which appear to lie within the ventricle, although they are
excluded from the cavity by the epithelium. When the tela is removed,
the delicate ependyma of the roof is torn away with it, leaving the line
of attachment to the stria medullaris to constitute the tænia thalami.
The =anterior wall= is formed by the =lamina terminalis= (s. cinerea), a
thin layer of gray matter which extends upward from the optic chiasm to
the corpus callosum. A distinct rounded band of white matter extends
across its posterior face, bulging into the ventricle. This is the
=anterior commissure= (Commissura nasalis) of the cerebrum; its fibers
extend to the olfactory bulb and to the pyriform lobe. A similar but
more slender =posterior commissure= (Commissura caudalis) crosses the
posterior wall above the entrance to the aqueduct; the connections of
its fibers are not yet clearly known. The =interventricular foramen= (of
Monroe) is situated on either side of the anterior part of the ventricle
and leads outward and slightly upward between the anterior pillar of the
fornix and the anterior tubercle of the thalamus. The cavity presents
three =recesses= or diverticula, of which two are ventral and the third
is supero-posterior. The =optic recess= (Recessus opticus) lies above
the optic chiasm. Just behind it is the =infundibular recess= (Recessus
infundibuli) which extends through the infundibulum to the pituitary
body. The =pineal recess= (Recessus pinealis) is in the stalk of the
pineal body.