Occipital Intralobar fasciculi: a description, through tractography, of three forgotten tracts

Diffusion MRI paired with tractography has facilitated a non-invasive exploration of many association, projection, and commissural fiber tracts. However, there is still a scarcity of research studies related to intralobar association fibers. The Dejerines’ (two of the most notable neurologists of 19th century France) gave an in-depth description of the intralobar fibers of the occipital lobe. Unfortunately, their exquisite work has since been sparsely cited in the modern literature. This work gives a modern description of many of the occipital intralobar lobe fibers described by the Dejerines. We perform a virtual dissection and reconstruct the tracts using diffusion MRI tractography. The dissection is guided by the Dejerines’ treatise, Anatomie des Centres Nerveux. As an accompaniment to this article, we provided a French-to-English translation of the treatise portion concerning five intra-occipital tracts, namely: the stratum calcarinum, the stratum proprium cunei, the vertical occipital fasciculus of Wernicke, the transverse fasciculus of the cuneus and the transverse fasciculus of the lingual lobule of Vialet. It was possible to reconstruct all but one of these tracts. For completeness, the recently described sledge runner fasciculus, although not one of the Dejerines’ tracts, was identified and successfully reconstructed.

normal brains fixed in the bichromate -that the stratum calcarinum 7 completely isolates the cuneus from the rest of the hemisphere, and that it opposes a barrier to the projection fibres, of which the cuneus would thus be deprived 8 . Microscopic examination of occipital lobe sections, stained by the Weigert or Pal methods, shows how inadequate the macroscopic sections are for determining the path of a bundle. Here, as in all the other regions of the cortex, we see fibres radiating from the cortex of the calcarine fissure crossing the layer of U-shaped fibres perpendicularly, or somewhat obliquely, and then [crossing] the association fibres of varying length, to contribute to the layer of the projection fibres, which are always situated deep and close to the ventricular cavities.
The systematic study of the limited lesions of the cuneus and the degeneration that they cause shows that (v. Monakow, Moeli, Henschen, Zinn, Vialet) the cuneus has projection fibres just like the other regions of the cerebral cortex, and that these fibres arrive at their destination by the shortest path, that is to say, by crossing the U-shaped fibres and the association fibres of varying length, before arriving to the layer of sagittal fibres of the occipital lobe .
The vertical occipital fasciculus or perpendicular occipital fasciculus of Wernicke (Ov), (fig. 377, 388, 389), stratum proprium convexitatis of Sachs , is a thick layer of fibres specific to the occipital lobe which connects the superior edge of this lobe to its inferior surface. It therefore connects th e superior occipital gyrus 9 with th e inferior occipital gyrus 10 , this la yer thins anteriorly; it connects the angular gyrus 11 to the middle and inferior temporal gyri 12 and thickly covers the thin layer of the fibres belonging to the parallel sulcus.
Behind the supramarginal gyrus, the vertical occipital fasciculus of Wernicke merges with the posterior or descending fibres of the superior longitudinal or arcuate fasciculus of Burdach.
As a whole, the fibres of the occipito-frontal fasciculus 13 thus constitute a kind of vertical septum extending from the occipital pole to the posterior branch of the Sylvian fissure. This vertical septum is traversed by the numerous fibres that enter into inferior longitudinal fasciculus, optic radiations 14 of the occipital-temporal lobe and the tapetum ; it is further crossed by the fibres of the transverse fasciculus of cuneus and the lingual lobule. Due to these numerous intersections, this layer of vertical fibres is poorly known laterally, it is on the contrary clearly delimited medially by the inferior longitudinal fasciculus, and its fibres are distinguished from those of the latter by their direction and their clearer coloring by hematoxylin stain.
The transverse occipital fasciculus of the cuneus 15 ( stratum cunei transversum of Sachs) ( ftcS) (fig. 290, 384 and 389) links the cuneus to the convexity of the occipital lobe and to its infero-lateral border; it belongs to the region of the cuneus, does not protrude past the parieto-occipital fissure, and has the same cortical origin as the stratum calcarinum. Its fibres arise from the upper lip of the calcarine fissure and project transversely and laterally; but instead of bending downwards like the fibres of the stratum calcarinum, they curl upwards, pass over the hollow cone formed by the occipital part of the inferior longitudinal fasciculus ,then pass through the vertical occipital fasciculus, intertwine with the commissural projection and association fibres of the region, and most likely radiate in the cortex of the convexity of the occipital lobe and its infero-lateral border. The most anterior fibres run a slightly obliquely, anteriorly and laterally 16 , and radiate in the superior parietal lobule and in the angular gyrus 17 (Sachs).
The transverse fasciculus of the lingual lobule of Vialet ( fig. 290 and 389, ftlgV) is to the lingual lobule what the previous fasciculus to the cuneus. Glimpsed by Sachs, well studied by Vialet, this fascicle is born from the lower lip of the calcarine fissure, projects transversely and laterally , and then inferiorly overlaps with the inferior longitudinal fasciculus. Its fibres are reflected at the level of the diverticulum of the lingual lobule and the medial basal bundle of Burdach, and a second time at the infero-lateral angle of the occipital horn, then they cross the vertical occipital fascic ulus of Wernicke, and radiate into the cortex of the convexity of the occipital lobe and its infero-lateral border. This fascicle connects the lower lip of the calcarine fissure to the convexity of the hemisphere. According to Vialet it represents the lower half of the association system that connects the calcarine region to the occipital convexity. In certain lesions of the occipital lobe it is sometimes found intact, among the degenerated The layer of fibres specific to the gyri of the medial surface of the hemisphere. -A layer of vertical fibres, similar to the fibres of the cuneus , is also found in the precuneus , in the precentral lobule and in the medial surface of the first frontal gyrus . These fibres, of varied length, arise from the upper edge of the hemisphere, project obliquely inferiorly and anteriorly, and terminate around the calloso-marginal and subparietal fissures. This layer of fibres extends from the cuneus to the frontal pole, and intersects with the numerous projection and commissural fibres which approach the gyri of the medial surface of the hemisphere; thanks to the oblique direction of its fibres, this layer appears darker on the vertico-transverse sections of brains hardened in the dichromate ( fig. 239 to 260), and is therefore easily distinguished from callosal radiations and fibres of the corona radiata, which circumvent the roof of the lateral ventricle and whose fibres are cut parallel to their length, as clearly shown by the microscopic vertical-transverse sections.
It is this layer of fibres specific to the gyri of the medial surface of the hemisphere, which Brissaud designated by the name of compact fascicle and diffuse fascicle of the fornix . There can be no question in this case, as we have just seen, of a long association fibre, analogous to the cingulum and extending from the frontal pole to the cuneus, or even to the lingual gyrus. To apply the name of "tracts of the fornix" to the layer of short association fibres of the gyri of the medial surface of the hem isphere is, in our opinion, to use a confusing term. The term fornix is today universally applied to the cerebral trigone; its use prevented the term peripheral fornix, applied by Arnold to the cingulum, from prevailing . Yet, we distinguish in the cerebral trigone the body of the fornix, the columns of the fornix, the pillars of the fornix; the works of Gudden, Forel, Honegger, etc., have shown that the cerebral trigone contains a very complex system of fibres: in addition to the fornix itself , there is described a fornix transversus , a fornix longus direct and crossed, a fornix obliquas , ect. For all these reasons, we will therefore reserve the term of fornix for the cerebral trigone and designate the layer of association fibres referred to her e a s " the layer of fibres specific to the gyri of the medial surface of the hemisphere".
In the frontal lobe , the system of specific association fibres is much less developed than in the occipital lobe. The occipital horn, around which the sagittal layers and the different layers of the short association fibres are grouped, extends far into the occipital lobe, while the frontal horn hardly extends beyond the anterior extremity of the caudate nucleus. Nevertheless, on the coronal sections, immediately anterior to of the head of the caudate nucleus and around the subependymal gray matter (Sge), which circumscribes the frontal horn (Vf) anteriorly, an annular layer arrangement, quite similar in appearance, though much reduced, to what is observed in the occipital horn (Fig.  390). The inner ring is formed within, above and below by the callosal fibres emanating from the genu (Ce), it is completed outside by the occipito-frontal fasciculus (OF). Around this first system of fibres, there is the irregular and incomplete ring formed by the fibres of the corona radiata of the frontal lobe (CR) . These fibres are arranged in a thick layer laterally, superiorly and inferiorly, and thin medially, where they are reinforced by the fibres of the cingulum, immediately, anterior to the genu of the corpus callosum. The fibres of the orbital and outer portions of the corona radiata are sectioned perpendicular to their length. The fibres of the upper parts of the corona radiata are cut more or less obliquely or parallel to their axis, they radiate towards, in fact, the medial and superior surfaces of the first frontal gyrus and in the cingulate gyrus 18 (Fig. 390).
This arrangement in sagittal layers is not found on horizontal sections (Fig. 391,296), the latter show, on the contrary, that the callous fibres are reflected anteriorly to the lateral ventricle before radiating towards the third frontal gyrus, and that the fibres of the corona radiata suggest a curve in the opposite direction around the callous fibres. The arrangement in concentric rings is due only to a false appearance, obtained thanks to the simultaneous section of the fibres of the genu of the corpus callosum and its radiating fibres.
It is around the fibres of the corona radiata that the layers of association fibres specific to the frontal lobe are grouped: some take a transverse direction and connect the medial surface of the frontal lobe to its orbital and lateral surfaces; the others have a vertical direction and provide the connections, either between the different gyri of its medial surface, or between the gyri of its orbital and super-external sides. Lastly, others affect in a sagittal direction; they are particularly numerous in front of the anterior perforated space and intersect with the anterior extremities of the fibres of the uncinate fasciculus, which radiate in the orbital surfaces of the first and third frontal gyri.
But these short association fibres are not arranged in compact layers, as in the occipital lobe -they intersect with the numerous projection and commissural fibres of the region. In front of the subependymal gray matter, the layers of differentiated fibre rapidly lose their individuality, such that most of the white matter of the frontal lobe is formed by the intimate entanglement of the association fibres with the commissural and projection fibres. It is along the olfactory sulcus, in the gyrus rectus and the orbital part of the first frontal gyrus, that we can best follow the aforementioned fibres (Figs 242 and 243, pp. 442 and 445). Le stratum calcarinum ( fig. 290 et 389, strK) est une épaisse couche de fibres verticales, qui double l'écorce de l'ergot de Morand, la sépare du faisceau longitudinal inférieur et forme à elle seule la masse blanche des plis cunéo-limbiques. Ses fibres relient la lèvres supérieure de la scissure calcarine à sa lèvre inférieure; les plus courtes et les plus superficielle unissent les partie profondes des deux lèvres de la scissure calcarine, les fibres les plus longues relient la face interne du cunéus à la face inféro-interne du lobule lingual. Cette couche de fibres verticales s'étend du pôle occipital au lobe limbique, unit au niveau de la branche commune aux scissures calcarine et pariéto-occipitale, la deuxième circonvolution limbique (circonvolution de l'hippocampe) au pli rétro-limbique de Broca, et renforce dans cette région le faisceau postérieur du cingulum. Elle représente en d'autres termes, la couche des fibres en U de la scissure calcarine.