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The external capsule is a series of white matter tracts in the brain situated between the putamen and claustrum. It is composed of claustrocortical fibers dorsally and the combined mass of the uncinate fasciculus and inferior frontal occipital fasciculus ventrally.
The putamen separates the external capsule from the internal capsule medially and the claustrum separates it from the extreme capsule laterally. Ventrally, the claustrum is deficient, and thus the boundary between the extreme capsule and external capsule is poorly defined.
The claustrocortical fibers of the dorsal extreme capsule appear to emanate radially from the periphery of the dorsal claustrum. These fibers pass through the corona radiata to reach to the frontal lobe and parietal lobe (including the supplementary motor area).
On their way through the corona radiata, they run laterally to the dorsal part of the frontal horn, body and the ventral atrium of the lateral ventricle. It should be noted that the claustrocortical fibers are bidirectional, that is, they are composed of fibers both emanating from and converging on the claustrum.
Inferior frontal occipital fasciculus and uncinate fasciculus
The inferior frontal occipital fasciculus and uncinate fasciculus together form the ventral external capsule. They pass through the temporal stem to the frontal lobe from the temporal lobe.
The uncinate fasciculus lies just posterior to the limen insulae in a hook-like shape ("uncinate" means "hook") and connects the temporopolar area to the orbital frontal lobe. It is sometimes characterized as having two parts, a dorsolateral and a ventromedial part, the latter also connecting to the septal area. On its way to the limen, it envelopes the inferior and anterior aspect of the nucleus accumbens.
The inferior frontal occipital fasciculus lies dorsal to the uncinate fasciculus and is a long association fiber tract, coursing from the ventral parts of the middle and inferior frontal gyri, through the limen and under the superior and middle temporal gyri to reach the middle and inferior occipital gyri. In the temporal lobe, it runs superior to the inferior longitudinal fasciculus.
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