The fornix crura, or better known as the crus or crura of fornix, are flattened, fibrous bands that are closely attached to the bottom portion of the corpus callosum.

The corpus callosum is a flat, wide bundle of neural fibers located underneath the cortex (outer layer of the brain) at the midline. The fornix is the main structural component of the hippocampal formation, which plays an important role in memory formation and recall.

The crura extend from the body of the fornix in a backward direction. Once the fornix crura split from one another, each part curves around the thalamus at the posterior (back) end, passing forward and downward into the lateral ventricle's inferior cornu. (The thalamus is a small structure in the brain that serves many functions including relaying motor and sensory signals and regulating sleep and wakefulness. Lateral ventricles are cavities within the brain that contain cerebrospinal fluid.)

The fornix crura then lie along the hippocampus' concavity, on the surface where some of the fibers spread out to form the alveus, which transmits information to the thalamus. The remaining fibers continue along as a white, narrow band — called the fimbria hippocampi — and become the hippocampal gyrus' uncus. The uncus is the hippocampal gyrus' anterior (front) extremity.

The thalamus, hippocampi, corpus callosum, fimbria, uncus, and lateral ventricle all extend outward from the fornix crura.