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White matter medial aspect

image ampoule pour afficher les régions

First focus on non-cortical structures.

Telencephalic commissures:

  • corpus callosum made of, from rostral to caudal: rostrum (2), genu (3), body (1), isthmus (4) and splenium (5),
  • fornix (8), caudally attached to the corpus callosum and rostrally connected to it by the septum pellucidum (6),
  • anterior commissure (11).

Lateral (telencephalic) ventricle, laterally limited by the caudate nucleus (7) and connected to the third ventricle by the interventricular foramen (13).

Diencephalic structures:

  • thalamus (14), limited from the fornix (8) by the velum interpositum (9),
  • hypothalamus (15), ventrally bordered by the mammillary body (10) and optic chiasm (12).

Mesencephalon: tegmentum (16) anteriorly and quadrigeminal plate (20) posteriorly.

Metencephalon, from anterior to posterior:

  • pons (17),
  • fourth ventricle (19),
  • cerebellum : superior medullary velum (23), vermis (22), cerebellar hemisphere (21).

Myelencephalon or medulla oblongata (18).

Focus on the sulco-gyral structures.

Main sulci of the medial aspect of the hemisphere:

  • cingulate sulcus (1), dorso-posteriorly continued by its marginal segment (2),
  • upper end of the central sulcus (3), better seen from the lateral aspect,
  • subparietal sulcus (4),
  • parieto-occipital fissure (5), that branches anteriorly with the calcarine fissure (6).

These main sulci limit lobes:

  • frontal lobe: medial aspect of the superior frontal gyrus (7, or F1), limited by the cingulate (1, 2) and central (3) sulci,
  • cingulate gyrus (8): limited by the cingulate (1), subparietal (4) and continued by its isthmus (9),
  • parietal lobe: medial aspect of the superior parietal lobule, or precuneus (10, or P1), limited by the central (3) and subparietal sulci (4), and parieto occipital (5) and calcarine (6) fissures,
  • occipital lobe : cuneus (11, or O6) dorsal to the calcarine fissure (6), and lingual gyrus (12, O5) ventral to it,
  • temporal lobe : medial aspect (13).

Accessory sulci are located within these lobes:

  • frontal : supraorbital sulcus (14), dorsally limiting the gyrus rectus (15),
  • occipital :
    • paracalcarine sulcus (16) within the cuneus (11),
    • lingual sulcus (17), within the lingual gyrus (12).

The paracentral lobule (18) is the superior pli de passage between the frontal and parietal lobes, around the superior tip of the central sulcus (3).

Remove the spongy cortex from the sulci using curettes and surgical suction.

Identify the sulci bordered by short "U" association fibers connecting both banks of sulci:

  • callosal sulcus (2), containing the indusium griseum (3),
  • central (12) and supraorbital (11) sulci,
  • cingulate sulcus (1), continued by its marginal segment (4),
  • subparietal sulcus (5), and parieto-occipital fissure (6),
  • calcarine fissure (7), paracalcarine (10), lingual (9) and occipitopolar (13) sulci,
  • collateral sulcus (8) lateral to the head of the hippocampus (14).

Use forceps and wooden spatulas to remove the "U" fibers along the cingulate sulcus.

Separate the cingulum (2) from the corpus callosum

Two contingents of fibers are visible:

  • callosal radiations (1), originating from the corpus callosum and having a radiate course towards the edge of the hemisphere,
  • cingulum (2), more medial, and having a longitudinal course.

Recline the cingulum (1) from anterior to posterior, to clearly identify the callosal radiations (2).

Cut the cingulum dorsal to the splenium of the corpus callosum (9) and remove it. The isthmus of the cingulum (10) is left in place.

Cut the postero-ventral aspect of the thalamus (12), to show the body of the fornix (4) continued by the crus fornicis (5) and then by the fimbria (6).

Progressively remove the thalamus medial aspect, to isolate the mammillo-thalamic tract (7), connecting the thalamus to the mammillary body (1).

Examine the Papez circuit:

  • fimbria (6), crus fornicis (5), body (4) and column of the fornix (3), that ends on the mammilary body (1),
  • mammillo-thalamic tract (7), connecting the latter to the thalamus (12),
  • thalamus (12), connected back to the hippocampus (8) via the cingulum 10) and the parahippocampal gyrus (11).

Observe the choroid plexus (2), that enters the lateral ventricle from the choroid fissure, between the thalamus (12) and fornix (4).

 

Using a scalpel, gently limit a window within the ependyma at the junction of the head (1) and body (3) of the caudate nucleus.

Carefully remove the ependyma in this window.

At the same level gently scrape the caudate nucleus to reveal the thalamic radiations (2). The putamino-caudate bridges are the grey matter joining the caudate to the putamen and located between the thalamic radiations.

Using a scalpel, longitudinally cut the corpus callosum, to better observe the caudate nucleus.

Remove the entire ependyma and caudate nucleus, to show the thalamic radiations (1).

Remove

  • the corpus callosum, at the level of its junction to the caudate nucleus,
  • the fornix and choroid plexus,
  • the hippocampus,
  • the mammillo-thalamic tract and the mammillary body,
  • the medial aspect of the thalamus.

Observe the ependyma, which covers the atrium (3) posterior (4) and inferior (5) horns of the lateral ventricle.

The lateral limit of the anterior horn (1) and body (2) of the ventricle (caudate nucleus lined by ependyma) was removed at the previous steps, showing the thalamic radiations.

Remove the ependyma of the atrium and posterior horn of the lateral ventricle (very adherent to the underlying white matter).

Use forceps and wooden spatulas to show the tapetum (2), arising from the corpus callosum and located along the ependyma.

(1) Short “U” association fibers.