Where it is
- The inner (medial) parts of the body of the fornix are joined by a thin triangular lamina, the psalterium (lyra), which holds the crossing fibres. [1]
- The two crura (back pillars) of the fornix, flattened bands closely attached at first to the under surface of the corpus callosum, are joined to each other through the commissure. [1]
- From there each crus curves around the back end of the thalamus and passes down and forward into the temporal horn of the lateral ventricle, along the hippocampus. [1]
What it does
- Its fibres connect the two hippocampi across the midline. [1]
- The fornix is counted in the limbic system: a white matter structure connecting the hippocampus with other brain structures, particularly the mammillary bodies and the septal nuclei. [2]
- Seen whole, each hippocampus and its neighbouring regions form a banana-like curve, and the two sides are joined at their back ends by this commissure. [3]
- It belongs to the fornix, the main output tract of the hippocampus, which also carries acetylcholine from the medial septum to the hippocampus and the theta rhythm made by nerve cells in the septal nuclei. [1]
- Honest limit: the exact function and importance of the fornix as a whole are still not entirely clear, and no separate role is described for the commissure beyond joining the two hippocampi. [1]
Connections
- At the front the body of the fornix divides into two columns that arch down to the mammillary bodies; at the back, the crura joined by this commissure lead to the two hippocampi. [1]
- Besides the large commissures (the corpus callosum and the anterior and posterior commissures), the brain has smaller ones; the lyra or hippocampal commissure is listed among them, with the habenular commissure, the optic chiasm and the commissures of the superior and inferior colliculi. [4]
Inside it
- White matter: the commissural nerve fibres that run inside the thin psalterium. [1]
Other names
- Also called the hippocampal commissure; the sheet that holds it is the psalterium or lyra (Lyra Davidis), named because the two crura and the crossing fibres look like a lyre or triangular harp. [1]
Size
- On this model the commissure of the fornix measures about 31 mm front to back, 20 mm top to bottom and 49 mm side to side (the box it fits in, not its volume). [5]
- Centre of the commissure of the fornix compared with the centre of the left fornix: 18 mm further back, 12 mm closer to the midline; 22 mm apart in a straight line. [5]