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White matter lateral aspect

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Localize the central sulcus (1) and lateral fissure (2), that limit the different lobes:

The frontal lobe, anterior to the central sulcus (1) and dorsal to the lateral fissure (2).

  • it is subdivided by:
    • the superior (3) and inferior (4) frontal sulci, more or less horizontal, and connected at right angle respectively to the superior (5) and inferior (6) precentral sulci.
  • it contains:
    • the precentral gyrus (7), between the central (1) and precentral (5, 6) sulci,
    • the superior (8), middle (9) and inferior frontal gyri, limited by the superior (3) and inferior (4) frontal sulci; the inferior frontal gyrus is subdivided by the vertical and horizontal rami of the anterior part of lateral fissure in pars orbitalis (10) triangularis (11) and opercularis (12).

The parietal lobe, posterior to the central sulcul (1) and dorsal to the lateral fissure (2) , is subdivided by the post-central sulcus (13) continued by the intraparietal sulcus (14), in :

  • postcentral gyrus (15), between the central (1) and post-central (13) sulci,
  • superior parietal lobule (16), dorsal to the intraparietal sulcus (14),
  • inferior parietal lobule made of the supramarginal (17) and angular gyri (18) sometimes limited from each other by the sulcus intermedius primus,
  • the subcentral gyrus (19) is the pli de passage between the pre (7) and post central (15) gyri. It is located around the ventral tip of the central sulcus (1).

The temporal lobe is ventral to the lateral fissure (2) and is subdivided by the superior (20) and inferior temporal (21) sulci, in superior (22), middle (23) and inferior (24) temporal gyri.

The occipital lobe is more or less limited from the temporal lobe by the anterior occipital sulcus (25), which is often absent or limited to a small temporo-occipital notch.

  • It is subdivided by two longitudinal sulci converging to the occipital pole :
    • the superior occipital sulcus (26) that continues the intraparietal sulcus and ends perpendicularly as the transverse occipital sulcus,
    • the inferior occipital sulcus (27) that continues the inferior temporal sulcus.
  • These sulci limit the superior (28), middle or lateral (29) and inferior (30) occipital gyri. The middle occipital gyrus (29) is sometimes subdivided by a longitudinal middle occipital sulcus (31).

Detach the cortex at the junction between the inferior parietal lobule and posterior temporal lobe.

Look at the U-fibers runing from both banks of each sulcus (1).

Progressively resect these U-fibers (1), to show the underlying vertically oriented posterior segment of the superior longitudinal fasciculus (SLFp, 2).

Continue the resection of U-fibers, to see SLFp (2) endings:

  • ventraly : posterior part of the superior (3) and midle (4) temporal gyri,
  • dorsally : supramarginal (5) and angular (6) gyri.

Once the SLFp is fully dissected (1), progressively remove the temporal and frontal opercula to continue the dissection of the surrounding tracts :

  • ventrally, remove T2 and T3 white matter to discover the inferior longitudinal fasciculus,
    • made of anterior (2) and posterior (3) segments.
    • the visual word form area (VWFA, 4) is located at the junction of both ILF segments and the SLFp (1).
  • anteriorly, find the anterior segment of the SLF (SLFa, SLF III, 5), which is horizontally oriented and dorsal to the insula.
  • medially to the SLFp (1) and SLFa (5), find the deep segment of the SLF or arcuate fasciculus (AF, 6), that curves around the posterior part of the insula.

Completly resect the posterior and anterior segments of the SLF to show its deep long segment or arcuate fasciculus (AF, 1) running from the precentral area and posterior part of inferior frontal gyrus to the posterior part of the superior, middle and inferior temporal gyri.

Observe the anterior (2) and posterior (4) segments of the ILF, connected by the visual word form area (VWFA, 3).

Resect the posterior and ventral parts of the arcuate fasciculus (1) to show the Inferior Fronto-Occipital Fasciculus (IFOF, 5) located medially.

The latter is made of slightly antero-posteriorly ascending fibers forming the most lateral part of the stratum sagittale.

Observe : the anterior (2) and posterior (4) segments of the Inferior Longitudinal Fasciculus, linked by the Visual Word Form area (WVFA, 3).

Detach the insular cortex, to see the underlying white matter or extreme capsule (6) made of U-shaped insulo-insular and insulo-opercular fibers.

Observe :

  • the Arcuate Fasciculus, (1),
  • the Inferior Longitudinal Fasciculus anterior (2) and posterior (4) segments linked by the Visual Word Form Area (3),
  • the Inferior Fronto-Occipital fasciculus (5) forming the most lateral layer of the Sagittal stratum.

Focus on the dorsal half of the extreme capsule.

In this area, remove the superficial part of the extreme capsule (U-shaped insulo-insular fibers) to discover, from lateral to medial :

  • the deep, insulo-opercular fibers of the dorsal extreme capsule (3),
  • the dorsal claustrum (5), which is compact
  • the dorsal external capsule (6), medial to the claustrum (5) and having a radiate pattern.

Continue the dissection of white matter bundles :

  • Inferior Fronto-Occipital Fasciculus (1),
  • anterior (2) and posterior (4) parts of the Inferior Longitudinal Fasciculus.

Resect the remaining dorsal extreme capsule to show, from lateral to medial:

  • the dorsal, compact claustrum (5),
  • the dorsal external capsule (6) having a radiate pattern (claustro-cortical projection fibers), appearing at the dorsal limit of the claustrum (5)

Focus on the ventral extreme and external capsules, which contain :

  • the trunk of the Inferior Fronto-Occipital Fasciculus (1), which is dorsal,
  • the trunk of the Uncinate Fasciculus (UF, 3), located ventral and lateral to the previous one and joining the temporal to the frontal lobe,
  • the ventral claustrum (7), made of small islands of grey matter intermingled with the trunks of the IFOF and UF.

Continue the dissection of the anterior (2) and posterior (4) parts of the Inferior Longitudinal Fasciculus.

Continue the dissection of the main ventral fasciculi:

  • anterior and posterior fans of the Inferior Fronto-Occipital Fasciculus (1) connecting F2, F3 and the frontal pole to P1, O1, O2, O3.
  • frontal and temporal fans of the Uncinate Fasciculus (2),
  • trunks of IFOF and UF, located within the ventral external and extreme capsules,
  • Inferior Longitudinal Fasciculus (3).

Medial to the dorsal claustrum (4), dissect the dorsal external capsule (5), made of radiate, claustro-cortical fibers.

Remove some fibers of the external capsule to show the lateral aspect of the putamen (6).

The internal capsule (7) runs medial to the superior aspect of the putamen (6) and fuses with the extreme and external (5) capsules to form the corona radiata (8).

Continue to gradually individualize the putamen (5) and the internal capsule (4) located medially and dorsally continued by the corona radiata (6).

Observe:

  • the occipital (O1 O2 O3), parietal (P1) and frontal (F2, F3) projections of the Inferior Fronto-Occipital Fasciculus (1),
  • the frontal and temporal projections of the Uncinate Fasciculus (3),
  • the Inferior longitudinal Fasciculus (2).

Continue the dissection of the trunks of the Uncinate and Inferior Fronto-Occipital Fasciculi, intermingled with the ventral claustrum (2).

The putamen (1) now clearly appears, lateral to the internal capsule.

Continue the resection of the putamen to show the putamino-caudate bridges located between the fibers of the internal capsule (1).

Dissect the lateral (3) and medial (2) globus pallidus located medial to the ventral putamen.

Observe the corona radiata (4), which continues the internal external and exrtreme capsules dorsally.