366 Neuroanatomy: Draw It to Know It
Hippocampus: Anatomy
Here, we will draw a coronal view of the medial tempo-
ral lobe (ie, the antero-inferior limbic lobe) to learn the
anatomy of the hippocampus and anterior parahip-
pocampal g yrus. Before we begin our diagram, draw a
small coronal insert in the corner of the page of half of
the brain and upper brainstem. Include the temporal
horn of the lateral ventricle and the tentorium cerebelli,
and encircle the medial temporal lobe — this is the region
we will draw.
Next, begin our main diagram. Draw the pons and
midbrain and extend our drawing along the basal fore-
brain. Th en, include the temporal horn of the lateral
ventricle. Next, use the choroid fi ssure to separate the
temporal horn of the lateral ventricle from the neighbor-
ing subarachnoid space: the ambient cistern.
To draw the hippocampus, fi rst draw a double-sided S.
Th en, divide the S into its superior turn, horizontal
stretch, and inferior turn. As we will show, the cornu
ammonis, which is Latin for “horn of the ram,” and which
is also known as Ammon’s horn or the hippocampus
proper, comprises the superior turn; the subicular com-
plex forms the horizontal stretch; and the parahippocam-
pal g yrus comprises the inferior turn. Internal to the
superior turn, draw the C-shaped dentate g yrus, which
cups the cornu ammonis. Next, label the subdivisions of
the cornu ammonis: label CA4 adjacent to the dentate
g yrus; label CA3 along the medial vertical; label CA2
along the top of the turn; and fi nally, label CA1 along the
lateral vertical. CA1 forms the largest stretch of Ammon’s
horn and is distinctively susceptible to anoxic injury.
Now, label the horizontal stretch of our diagram as
the subicular complex, and show that the hippocampal
sulcus separates it from the dentate g yrus. From lateral to
medial (ie, from CA1 to the ambient cistern), the subic-
ular complex comprises the subiculum, presubiculum,
and parasubiculum. Th e subiculum is the source of the
postcommissural fornix fi bers, which terminate in the
mammillary nuclei and which form a key step in the Papez
circuit. Th e Terminologia Anatomica lists the complete
subicular complex as part of the hippocampus, whereas
other sources list the subiculum, only, as part of the hip-
pocampus and list the pre- and parasubiculum as part of
the parahippocampal region.
Next, label the inferior turn as the entorhinal cortex,
which forms the medial aspect of the anterior parahip-
pocampal g yrus. Now, draw the perirhinal cortex and
show that the rhinal sulcus separates the perirhinal and
entorhinal cortices. Next, in the corner of the diagram,
write out that the entorhinal and perirhinal cortices
form the anterior parahippocampal g yrus. Th e posterior
parahippocampal g yrus is referred to as the parahip-
pocampal cortex and corresponds to Von Economo areas
TF and TH.^11 Next, for anatomic context, show that the
collateral sulcus distinguishes the fusiform g yrus, later-
ally, from the parahippocampal g yrus, medially.
Now, let’s discuss the uncus, which, simply put, forms
the anterior g yral thumb of the parahippocampal g yrus.
Anteriorly, the uncus encompasses the amygdala, and
posteriorly, it forms the anterior portion of the hip-
pocampus, which complicates the simple defi nition of
the uncus as listed, here. Th e term “uncal herniation” is
used to describe medial temporal lobe herniation over
the tentorium cerebellum. During uncal herniation, the
medial temporal lobe compresses the ipsilateral third
nerve as it exits the midbrain, causing an ipsilateral ocul-
omotor palsy. Also, the herniating temporal lobe can
compress the contralateral cerebral peduncle against its
adjacent tentorium, forming a so-called Kernohan’s
notch in that peduncle. Th e damaged cerebral peduncle
results in motor weakness on the opposite side of the
body — the side of the herniating temporal lobe and
oculomotor palsy.
Finally, internal to the perirhinal and entorhinal cor-
tices, label the parahippocampal white matter; and then
label the white matter along the lateral border of the
cornu ammonis as the alveus; and show that along the
superomedial border of the cornu ammonis, the alveus
becomes the fi mbria.^3 , 6 – 9 , 12