The fornix is a C-shaped bundle of nerve fibres deep in the middle of the brain, one on each side: it arches from the hippocampus in the temporal lobe up and forward under the corpus callosum, then down in front of the thalamus to the mammillary bodies. It is the major output tract of the hippocampus. [1]
The 3D brain needs JavaScript. The text below still says where the fornix is and what it does.
Slice
Opens cut by a side (sagittal) slice through the fornix. Drag to turn, pinch or scroll to zoom, tap any part to name it; .
It starts in the temporal horn of each lateral ventricle, where fibres spread over the hippocampus (the alveus) and gather into a narrow white band, the fimbria of the hippocampus. [1]
From there each crus (back pillar) curves up around the back end of the thalamus and runs forward, its start closely attached to the underside of the corpus callosum. [1]
The two crura join under the corpus callosum to form the body of the fornix, which runs forward along the midline. [1]
At the front it splits into two columns that arch down in front of the interventricular foramina and behind the anterior commissure, then descend through the lateral wall of the third ventricle to the base of the brain, where they end in the mammillary bodies. [1]
What it does
It is the main output pathway of the hippocampus: it carries what the hippocampus has processed on to deep structures of the brain. [1]
It also carries fibres the other way: acetylcholine, which is crucial for encoding memories, reaches the hippocampus from the medial septum through the fornix, and so do the theta rhythms generated by neurons in the septal nuclei. [1]
It is essential for acquiring and consolidating new episodic memories (memories of events), and is most closely tied to recalling information rather than recognising it. [1]
It is the second link of the Papez circuit: hippocampus → fornix → mammillary bodies → anterior thalamic nuclei → cingulate gyrus → parahippocampal gyrus → back to the hippocampus. [2]
Honest limit: its exact function and importance are still not entirely clear. [1]
Connections
Its back (postcommissural) fibres run through the hypothalamus to the mammillary bodies, which pass the signal on to the anterior nuclei of the thalamus through the mammillothalamic tract. [1][3]
Its front (precommissural) fibres end in the septal nuclei of the basal forebrain and the nucleus accumbens. [1]
A thin triangular sheet between the two halves, the psalterium (lyra), carries commissural fibres that join the left and right hippocampi across the midline. [1]
Inside it
White matter: a tract of nerve fibres, made of four parts in order, the fimbria, the crura, the body and the columns. [1]
Other names
“Fornix” is Latin for “arch”; the plural is fornices. It is also called the fornix cerebri. [1]
The joining fibres between the two sides are the commissure of the fornix, also called the hippocampal commissure; the sheet that holds them is the psalterium or lyra (Lyra Davidis), named for its harp-like shape. [1]
Size
On this model the left fornix measures about 45 mm front to back, 32 mm top to bottom and 31 mm side to side (the box it fits in, not its volume); the right one is its mirror image. [4]
Centre of the left fornix compared with the centre of the left hippocampus: 15 mm higher, 15 mm closer to the midline; 21 mm apart in a straight line. [4]
Centre of the left fornix compared with the centre of the corpus callosum: 5 mm further back, 14 mm lower, 12 mm further from the midline; 19 mm apart in a straight line. [4]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.