364 Neuroanatomy: Draw It to Know It
Limbic System, Part 2 ( Advanced )
Now, let’s return to the subcallosal region and draw a
detailed view of its anatomy; include the anterior com-
missure for reference. Indicate that the anterior paraol-
factory sulcus separates the subcallosal g yrus from the
frontal lobe, anteriorly, and show that the posterior
paraolfactory sulcus divides the subcallosal area into the
subcallosal g yrus, anteriorly, and the paraterminal g yrus,
posteriorly. Note that additional descriptors for this
region include the terms the septal area and paraolfactory
(or parolfactory) area. Also note that the nomenclature
used to describe the subcallosal region displays extraordi-
nary intertextual inconsistency and contradiction. Next,
show that the septal nuclei lie, most notably, within the
fi brous septum pellucidum above the anterior commis-
sure, beneath the corpus callosum, posterior to the sub-
callosal area, and anterior to the fornix. Th en, show that
the bed nucleus of the stria terminalis sits just above the
anterior commissure and below the septal nuclei.
Next, let’s draw the limbic pathways in sagittal view.
Draw an outline of a sagittal view of the limbic system
gray matter, corpus callosum, and thalamus. Th en, show
that the cingulum spans the cingulate and parahip-
pocampal g yri. Indicate that it interconnects intracingu-
late areas; synapses in the hippocampus (through the
entorhinal cortex); and also projects to many additional
areas, including most cortical regions, including the pre-
frontal, temporal, parietal, and occipital cortices. Next,
show that the fornix emerges from the hippocampus,
wraps around the thalamus, and passes anteriorly just
below the corpus callosum. At the interventricular fora-
men of Monro it divides and descends as bilateral col-
umns. Each column contains a small precommissural
bundle that descends anterior to the anterior commis-
sure and projects to diff use areas, including the septal
nuclei, frontal lobes, hypothalamus, and ventral striatum;
and each column also contains a large postcommissural
bundle, which originates solely within the subiculum of
the hippocampus, projects posterior to the anterior
commissure, and synapses, most notably, in the mammil-
lary bodies. Note that postcommissural fornix connec-
tions to the anterior thalamic nuclei and midbrain also
exist. Next, show that the mammillothalamic fasciculus
connects the mammillary body to the anterior thalamic
nuclei. Note that the postcommissural fornix and the
mammillothalamic fasciculus form key steps in the Papez
circuit.
Now, show that via the stria terminalis, the amygdala
projects along the dorsal aspect of the thalamus and ter-
minates in the septal nuclei and bed nucleus of the stria
terminalis and also in the hypothalamus. Finally, show
that via the amygdalofugal tract, the amygdala projects
to diff use areas: most notably, the basal forebrain, pre-
frontal, temporal, occipital, and insular cortices; thala-
mus; hypothalamus; septal nuclei; and the autonomic
and neurobehavioral substrate of the brainstem.^8
Finally, because the stria terminalis and fornix follow
such similar paths, let’s draw a coronal view of the dien-
cephalon to distinguish their trajectories; in this dia-
gram, we will also show the position of the stria medullaris
thalami, which is an important diencephalic pathway
that travels near the fornix and stria terminalis. As ana-
tomic landmarks, draw the corpus callosum, caudate,
thalamus, lateral ventricle, and third ventricle. Now,
show the body of the fornix underneath the midline of
the corpus callosum; the fornix emerges from the hip-
pocampus, wraps around the thalamus, and passes ante-
riorly in midline just below the corpus callosum. Next,
draw the stria terminalis along the inferomedial aspect
of the caudate (lateral to the body of the fornix); the stria
terminalis emerges from the amygdala and passes along
the inferomedial aspect of the caudate nucleus through-
out most of its course along the lateral wall of the lateral
ventricular system. Finally, in midline, along the dorso-
medial aspect of the thalamus, draw the stria medullaris
thalami, which spans from the habenula to the septal
nuclei.^3 , 6 – 10