Here you can see the septum
pellucidum extending between
the corpus
callosum and
fornix.
As you will see
in later plates, the septum
pellucidum a membrane that separates the two lateral
ventricles through much of
the anterior cerebrum.
No doubt, on one of your sagittal sections you will have an intact septum pellucidum and on the other
you should be able to see between the
corpus callosum and the fornix
into the lateral
ventricle.
It's often instructive to take your probe and rip an intact sepum pellucidum so you can tear
away the veil of the lateral ventricle
and look inside.
Just below where the "C" curve of
the fornix fuses into the
brain, you can see the small point of the anterior commisure.
Below that, on the ventral
surface of the brain, the large white mass
is the optic
chiasm. Following
caudally along
the ventral surface of the
brain you will see another
round mass that is made up of gray matter. This is a
mammillary
body. The mammillary
bodies are the main
targets of the fibers of the fornix,
which had their origins in the hippocampi, which cannot be
seen in
this section.
About 2/3 of the way to the caudal end of this section,
you can see another membrane hanging form the caudal end of the inferior
colliculus and extending
under the rostral end of the cerebellum.
This is the anterior medullary vellum.
To see this membrane clearly, you may have to use your probe to gently
lift it off of the dorsal
surface of the brain stem. The anterior medullary vellum, like the
septum pellucidum, is related
to the ventricular system. The anterior
medullary vellum seals cerebospinal fluid below it as
the fluid
flows caudally out of the cerebral
aqueduct into the fourth
ventricle, which runs along the dorsal surface of the pons and
part of the medulla.
The membranes
forming the roof of the caudal fourth
ventricle are "more complicated," and we'll leave it at
that.
If this section were directly on
the midline, you could see through the full extent of the cerebral aqueduct. Here, the
aqueduct is open through only
half of its course through the midbrain. If you have a
brain like this, you might gently probe rostrally through the aqueduct
and you may be able to see the probe entering the third ventricle
at
the caudal end of the thalamus.