The Fornix

The fornix is a strange structure – boring as hell and yet highly exciting. It seems boring as long as you only ask about its function. After all, it is merely a bundle of strands, a fiber system, a collection of axons from nerve cells that connect three widely separated centers of the cerebrum: the hippocampi, the septal nuclei, and the mammillary bodies. A bundle of fibers, white matter – nothing more. Nevertheless, it is a powerful bundle of fibers in the limbic system, to which the three centers mentioned above belong.

When you look at it, the fornix gains immensely: its size alone is impressive. This is due to the distance between the connected structures. And then there is its shape: the fornix does not consist of straight lines – oh no! It has curves and arches like a Gothic building. In fact, its shape is reminiscent of the self-supporting ribs of a cross vault, which made it possible to construct mighty cathedrals with high ceilings. Such a vault spans the middle of the forebrain. And that is exactly how the name fornix can be translated into English: vault. But the structure has also been described using many other vivid terms.

The fornix thus resembles the curved X of the stone ribs in a vault. At the four ends are structures that are connected to each other via the nerve fibers of the fornix. The two rear ends of the fornix are connected to the hippocampi of the right and left temporal lobes via the fimbriae hippocampi (the fimbria of hippocampus). These fimbriae merge into the right and left crus fornicis (crus of fornix), the two legs of the fornix. These two branches initially point backward, but then curve upward, forward, and inward, rising to the intersection point of the X, i.e., to the keystone of the vault, so to speak.

This intersection point, located in the middle of the forebrain, directly below the corpus callosum, is also called the corpus fornicis. There, you find many nerve fibers that connect the right and left hippocampus reciprocally via the fornix branches, the commissura fornicis. Before the intersection point, the fornix divides again, this time into the right and left columna fornicis, the fornix columns. These two columns turn sharply downward. Each of these divides again into the thinner anterior fornix column, which descends toward the septal nuclei, and the more powerful posterior fornix column, which continues downward and even backward toward the mammillary bodies of the hypothalamus. Nerve fibers originating from nerve cells in the septum run through the anterior fornix column to the hippocampus. The posterior fornix column consists mainly of fibers that run from nerve cells in the hippocampus to the mammillary bodies.

In the central longitudinal section through the brain, only a small part of the fornix is visible: its center, the intersection point, the keystone, the corpus fornicis. However, this visible part actually spans like a vault over the third ventricle of the diencephalon and the thalami flanking it. And above this spans the equally beautifully arched and curved commissure, the corpus callosum.

In very simple terms, it can be said that the fornix mainly contains the outputs of the hippocampal formation, most of which extend to the opposite hippocampus and to the mammillary bodies. It is an important pathway in the Papez circuit. Complete destruction of the fornix – which fortunately almost never occurs – leads to symptoms similar to those of destruction of the hippocampus: loss of memory functions.

First published on August 23, 2011
Last updated on July 1, 2025