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I have recently been trying to understand the three dimensional nature of the brain for a neuroanatomy class. Part of this strategy has included learning about structures such as the falx cerebri, tentorium cerebelli, anterior dural girdle, etc in order to understand the physical barriers that surround different parts of the brain and therefore separate them from one another. For example, the reason that the cerebellum and floor of the occipital lobe are not directly continuous is because of the tentorium cerebelli.

One issue that I have recently come across is understanding why the inferior medial component of the frontal lobe, in the region just anterior to the pituitary gland and optic chiasm, are not directly continuous (i.e. the left cortical hemisphere at this location is not continuous with the right cortical hemisphere). I have attached some photos to clarify my confusion:

Picture of Falx Cerebri

As you can from this image, the falx cerebri's anterior extent leaves a small opening (indicated by the red line) that would, presumably, permit grey matter from the left hemisphere to be directly continuous with grey matter from the right hemisphere. However, no such connection exists. This is readily confirmed using an inferior view, such as that indicated in the following picture, boxed in red:

Ventral View of Brain

As you can see from the boxed area, the medial inferior cortical grey masses from the left and right hemisphere (which are likely the subgenual division of the anterior cingulate gyrus) are distinctly separate. This separation is clearly not related to the falx cerebri, as the anterior descent of the falx cerebri occurs further in front of this area (as indicated by picture 1).

Could anyone offer an explanation as to why these structures are distinctly NOT continuous at this point? Further, if anyone could confirm whether any part of the cortex exhibits grey-matter-continuity (i.e. not via white matter tracts) between the left and right hemisphere, that would be greatly appreciated.

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Could anyone offer an explanation as to why these structures are distinctly NOT continuous at this point?

The crista galli physically separates the straight gyrus.

Crista Galli Click to zoom

Straight Gyrus Click to zoom

... that would, presumably, permit grey matter from the left hemisphere to be directly continuous with grey matter from the right hemisphere. However, no such connection exists.

They are not physically touching in that location, elsewhere the two hemispheres are not connected (even though physically touching) by gray matter.

Further, if anyone could confirm whether any part of the cortex exhibits grey-matter-continuity (i.e. not via white matter tracts) between the left and right hemisphere, that would be greatly appreciated.

Except in rare cases, for example agenesis of the corpus callosum (ACC), communication between the hemispheres is via the anterior commissure and the corpus callosum.

The two halves are separated by a deep longitudinal fissure, see this animation:

Longitudinal Fissure

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  • $\begingroup$ sorry, but I do not believe the crista galli separates the WHOLE straight gyrus...it only separates the anterior portion, by virtue of the falx cerebri, which is illustrated in the very first picture I posted. $\endgroup$ – S.Cramer Mar 24 at 21:40
  • $\begingroup$ That's OK. I didn't claim that it does nor do my images show that, as explained in my second paragraph it's not the only factor; watch the animation, where it pauses. $\endgroup$ – Rob Mar 24 at 21:51

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