The fornix (Latin for "arch") is a C-shaped bundle of fibers (axons) in the brain that carries signals from the hippocampus to the mammillary bodies and then to the anterior nuclei of thalamus. The fornix is part of the limbic system. While its exact function and importance in the physiology of the brain is still not entirely clear, it has been demonstrated that surgical transection – the cutting of the fornix along its body – can cause memory loss in humans. There is some debate over what type of memory is affected by this damage, but it has been found to most closely correlate with recall memory rather than recognition memory. This means that damage to it can cause difficulty in recalling long-term information such as details of past events, but it has little effect on the ability to recognize objects or familiar situations.
The fibres begin in the hippocampus on each side of the brain (where they are also known as the fimbria); the separate left and right sides are each called the crus of the fornix. The bundles of fibres come together in the midline of the brain, forming the body of the fornix. The inferior edge of the septum pellucidum (a membrane that separates the two lateral ventricles) is attached to the upper face of the fornix body.