Where is the fornix?
The term fornix comes from Latin and means "arch." It is used to refer to various arch-like structures in the body, but when used in reference to the brain it indicates a bundle of white matter fibers that arches around the thalamus. The fornix originates in the hippocampus, where it emerges from a collection of fibers called the fimbria. It then stretches up and around the thalamus toward the front of the brain. When it reaches a tract called the anterior commissure, it branches downward. Some fibers then split off and terminate mainly in the septal nuclei, preoptic nuclei, and ventral striatum, while others enter the hypothalamus and form connections with the mammillary bodies.
What is the fornix and what does it do?
In 1937 the neuroanatomist James Papez described what came to be known as the Papez circuit. The Papez circuit consisted of a group of structures---including the hippocampus, mammillary bodies, anterior nucleus of the thalamus, cingulate gyrus, and parahippocampal gyrus---that Papez hypothesized were the "anatomic basis of emotions." The fornix was a critical component of the Papez circuit, acting as a primary connection among several structures within the circuit. The Papez circuit would later be expanded upon and termed the limbic system.
The diverse group of structures known as the limbic system is now thought to be involved in much more than emotion, and the fornix is still considered an important part of the limbic system. The fornix acts as the primary outgoing pathway from the hippocampus, and thus its most recognized function is its involvement in memory. The hippocampal projections that travel in the fornix are thought to be important for memory consolidation, and damage to the fornix has been associated with anterograde amnesia, which involves the inability to create new memories. Fornix damage is primarily linked to deficits in declarative memories, or memories for factual information---and especially episodic memories, which are a type of declarative memory that deals with autobiographical information.
Neurodegeneration in the fornix has also been associated with the cognitive impairment seen in Alzheimer's disease. The integrity of the fornix may be compromised in the early stages of Alzheimer's disease, and thus may be an early indicator of the disease that can predict the progression of Alzheimer's disease from preclinical (i.e. asymptomatic) to clinical (i.e. symptomatic) stages. Degeneration of the fornix in Alzheimer's disease seems to precede degeneration of the hippocampus, an area that is known to be severely affected by the disease.
Although the functions of the fornix are still relatively poorly understood, its role in memory processes seems to be one that is relatively well supported. Due to its diverse connections, the fornix likely is involved in a list of other brain activities, but more research will be needed to further elucidate these roles.