298 Neuroanatomy: Draw It to Know It
Commissures & Disconnections ( Advanced )
Here, we will draw the commissural fi bers and learn
about the major disconnection syndromes. Commissural
fi bers connect corresponding areas of the cerebrum; the
two main commissural bundles are the anterior commis-
sure and the corpus callosum. Th e corpus callosum lies
superior to the anterior commissure and connects dorsal
cortical areas, whereas the anterior commissure connects
ventral areas. First, let’s use an axial plane through the
inferior surface of the brain. Hash the anterior commis-
sure bundle, which connects the bilateral ventral poste-
rior frontal lobes and also the bilateral anterior temporal
lobes. Th en, draw the corpus callosum fi bers, which con-
nect the bilateral hemispheres from the frontal to the
occipital poles. Indicate that the frontal fi bers are called
the anterior forceps (aka forceps minor, frontal forceps)
and that the occipital fi bers are called the posterior for-
ceps (aka forceps major, occipital forceps).
Next, let’s draw a sagittal view of the corpus callosum
to show its four regions. Label the anterior-inferior por-
tion of the corpus callosum as the rostrum; the anterior-
superior portion as the genu, which is the anterior bend;
the length of the corpus callosum as the body; and the
posterior portion as the splenium. Indicate that the pre-
frontal fi bers run through the rostrum and genu; the
frontal fi bers run through the anterior body; the tem-
poro-parietal fi bers run through the posterior body; and
the temporo-occipital fi bers run through the splenium.
Now, return to our axial diagram and indicate that the
genu fi bers correspond to the anterior forceps; the sple-
nial fi bers correspond to the posterior forceps; and the
body fi bers correspond to the midportion of the callo-
sum. Th e rostral fi bers run beneath the genu and are infe-
rior to the plane of this axial section.
Additional smaller commissural fi ber pathways exist,
including the hippocampal commissure, which lies infe-
rior to the splenium of the corpus callosum and connects
the bilateral hippocampal formations, and the posterior,
habenular, and supraoptic commissures.
Th e function of the commissural fi bers has aroused
great interest throughout the past several centuries, and
although the commissural fi bers have long been under-
stood to provide interhemispheric communication, the
full extent of their function is still not known. Functional
analysis of the corpus callosum suggests a role for it in
both sensory integration and high-level cognitive pro-
cessing. Much of what is known about the commissural
bundles, however, comes from callosal resection surger-
ies, which are still done today to prevent the transmis-
sion of epileptic activity between the cerebral hemispheres
(ie, to stem the propagation of seizures), and also from
commissural fi ber disruption injuries, which can result
in disconnection syndromes.
Here, we will address two major disconnection syn-
dromes: pure alexia without agraphia and alien hand
syndrome. In 1892, Dejerine memorably described the
syndrome of pure alexia without agraphia, which is a syn-
drome in which patients are unable to read but are still
able to write. To illustrate the anatomic underpinnings of
alexia without agraphia, draw an anatomic axial section
through a mid-height of the cerebrum. Th en, label the
language center in the left superior temporal g yrus. Next,
label the left visual cortex and show that it directly com-
municates with the language center. Th en, draw the right
visual cortex and then the splenium of the corpus callo-
sum. Indicate that the right visual cortex projects to the
language center through the splenium of the corpus cal-
losum. Now, show that in pure alexia without agraphia,
there is a posterior cerebral lesion that aff ects the medial
left posterior occipital lobe and the related visual projec-
tion fi bers, and indicate that the lesion also involves the
right visual projection fi bers aft er they have crossed mid-
line within the splenium of the corpus callosum. From
this lesion, the left visual center is directly cut off from
the language center and the communication between the
right visual center and the language center is disrupted
from the injury to the corpus callosum.