Ambiguity surrounds the existence and morphology of the human forniceal commissure. We combine advanced in-vivo tractography, multidirectional ex-vivo fiber dissection, and multiplanar histological analysis to characterize this structure’s anatomy. Across all 178 subjects, in-vivo fiber dissection based on the Human Connectome Project 7T MRI data identifies no interhemispheric connections between the crura fornicis. Multidirectional ex-vivo fiber dissection under the operating microscope demonstrates the psalterium as a thin soft-tissue membrane spanning between the right and left crus fornicis, but exposes no commissural fibers. Multiplanar histological analysis with myelin and Bielchowsky silver staining; however, visualizes delicate cruciform fibers extending between the crura fornicis, enclosed by connective tissue, the psalterium. The human forniceal commissure is therefore much more delicate than previously described and presented in anatomical textbooks. This finding is consistent with the observed phylogenetic trend of a reduction of the forniceal commissure in non-human primates compared to nonprimate eutherian mammals.
Fig. 1 Commissural connections in relation to the fornix as identified with in-vivo tractography: Representative streamline reconstruction of the fornix (a) and in relation to its neighboring anatomy (b).Fig. 2 Fiber dissection from the ventral aspect: a. Exposure of the inferior longitudinal fasciculus (ILF) and the fibers of the parahippocampal gyrus (fPHG). b. View on the roof of the lateral ventricles (RLV), crus (crF), body (bF) and column (coF) of the fornix, and the psalterium (Ps). c. Exposure of the dentate gyrus (DG), fimbria (Fi), gyrus fasciolaris (GF), the subcallosal trigone (SCT), inferior surface of the uncal apex (iUA), band of Giacomini (BG), and the external digitations of the hippocampal head (ED). d-e. Dissection of the fornix from the dorsal end of the fimbria (Fi) along the crus (crF) towards the body of the fornix (bF). No commissural fibers between the fornicis were identified. Removal of the ependymal layer of the roof of the lateral ventricle (RLV) exposed the fibers of the body of the corpus callosum (BCC).Fig. 3 Fiber dissection from the caudal aspect and morphometry of the psalterium: a. Exposure of the splenium of the corpus callosum (SCC) and the quadrigeminal plate (QP). b. Closer view on the SCC. c. Dissection of the subcortical fibers of the right isthmus revealed the fibers of the forceps major (FM). d. Posterior oblique view on the crus fornicis (crF), the choroidal fissure (FC), and the pulvinar (Pu). e. Midline incision and removal of the right half of the SCC. f. Another specimen after fenestration of the SCC and partial removal of the thalamus (Th) to visualize the A, crF and psalterium (Ps). g. Separation and elevation of the medial part of the SCC from the Ps. h. The length (l) of the Ps was measured from the unification of the crF to form the bF to the SCC. The width (w) of the Ps was defined as the distance between right and left attachment of the crF to the SCC.Fig. 4 Multiplanar histology of the psalterium: (a) Schematic illustration of the histological section planes: coronal (B), sagittal (C) and axial (D). (b–d) Floating sections stained for myelin. b. Coronal sections from caudal to rostral. c. Sagittal sections from left to right. d. Axial sections from basal to dorsal. #
No conflict of interest has been declared by the author(s).
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Fig. 1 Commissural connections in relation to the fornix as identified with in-vivo tractography: Representative streamline reconstruction of the fornix (a) and in relation to its neighboring anatomy (b).Fig. 2 Fiber dissection from the ventral aspect: a. Exposure of the inferior longitudinal fasciculus (ILF) and the fibers of the parahippocampal gyrus (fPHG). b. View on the roof of the lateral ventricles (RLV), crus (crF), body (bF) and column (coF) of the fornix, and the psalterium (Ps). c. Exposure of the dentate gyrus (DG), fimbria (Fi), gyrus fasciolaris (GF), the subcallosal trigone (SCT), inferior surface of the uncal apex (iUA), band of Giacomini (BG), and the external digitations of the hippocampal head (ED). d-e. Dissection of the fornix from the dorsal end of the fimbria (Fi) along the crus (crF) towards the body of the fornix (bF). No commissural fibers between the fornicis were identified. Removal of the ependymal layer of the roof of the lateral ventricle (RLV) exposed the fibers of the body of the corpus callosum (BCC).Fig. 3 Fiber dissection from the caudal aspect and morphometry of the psalterium: a. Exposure of the splenium of the corpus callosum (SCC) and the quadrigeminal plate (QP). b. Closer view on the SCC. c. Dissection of the subcortical fibers of the right isthmus revealed the fibers of the forceps major (FM). d. Posterior oblique view on the crus fornicis (crF), the choroidal fissure (FC), and the pulvinar (Pu). e. Midline incision and removal of the right half of the SCC. f. Another specimen after fenestration of the SCC and partial removal of the thalamus (Th) to visualize the A, crF and psalterium (Ps). g. Separation and elevation of the medial part of the SCC from the Ps. h. The length (l) of the Ps was measured from the unification of the crF to form the bF to the SCC. The width (w) of the Ps was defined as the distance between right and left attachment of the crF to the SCC.Fig. 4 Multiplanar histology of the psalterium: (a) Schematic illustration of the histological section planes: coronal (B), sagittal (C) and axial (D). (b–d) Floating sections stained for myelin. b. Coronal sections from caudal to rostral. c. Sagittal sections from left to right. d. Axial sections from basal to dorsal.
