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Dissection Instructions:
Rostral Medial Face--
The relationship between the 3rd ventricle and the thalamus is somewhat
unusual. Most of the medial portions of the two thalami are fused in a
structure called the massa intermedia (appropriately named, if you remember
your latin roots). Where this fusion exists, of course, there can be no
ventricular space. Consequently, the 3rd ventricle must run around the
massa intermedia. In the vicinity of the massa intermedia, the lateral
walls of the 3rd ventricle are formed by the unfused medial nuclei of the
two thalami. Ventrally the lateral walls of the 3rd ventricle are formed
by the medial nuclei of the hypothalami. At about 10 o'clock from the massa
intermedia (assuming the brain is horizontal with the dorsal side up),
locate the small piece of tissue that is just outside the 3rd ventricle.
This is the pineal body. Looking at the most ventral part of the 3rd ventricle,
you should be able to appreciate how it extends down into the hollow stalk
attaching the pituitary and hypothalamus (the infundibulum). Just caudal
to this region, you will find the cut surface of the mammillary body. Just
rostral to this region, you will find the cut surface of the optic chiasm.
The corpus callosum is the very prominent collection of axons that extends
for some distance along the medial face of the cerebral hemispheres. At
the rostral end, it curves ventrally and caudally, virtually making a 180o
turn. The area of turning is called the genu, and if you look carefully
(with a perfectly cut brain), you will see that the caudal extending portion
of this bend comes to a point and ends; this is the rostrum. At the caudal
end of the corpus callosum
it can be seen that a similar 180o turn is made; this bend is the
splenium. The main "body" of the corpus
callosum runs between these two turns.