Axons of hippocampal neurons converge to form a prominent bundle that arches around caudal, superior, and rostral aspects of the thalamus. This bundle, the fornix, is a major efferent path of the hippocampal formation. It is composed of a flattened caudal part, the crus; a compact superior portion, the body; and a part that arches around the rostral part of the thalamus and passes through the hypothalamus to terminate in the mammillary body - this is the column of the fornix. Located along the edge of the dentate gyrus and continuing on the lateral edge of the crus and body of the fornix is a thin fringe of fibers called the fimbria.

The amygdaloid nuclear complex (commonly called the amygdala) is located internal to the cortex of the uncus. It is composed of several cell groups, including caudomedial, basolateral, and central subdivisions. Two major efferent bundles are related to the amygdala. First, the stria terminalis follows a looping trajectory that shadows, in a reverse direction, the orientation of the caudate nucleus. In the temporal horn, the stria terminalis is located just medial to the tail of the caudate nucleus. As the stria terminalis arches superiorly and rostrally, it assumes a position in the shallow groove between the caudate nucleus and the dorsal thalamus, where it is accompanied by the terminal vein (superior thalamostriate vein). At about the level of the interventricular foramen, the fibers of the stria terminalis fan out to enter and terminate in the hypothalamus, the septal area, and the neostriatum.

The second major efferent bundle of the amygdala is the diffusely arranged ventral amygdalofugal pathway. These fibers leave the amygdaloid complex, pass medially through the substantia innominata, and continue medially to enter hypothalamic and septal nuclei or turn caudally and distribute to the brainstem.

Cell groups located internal to the subcallosal area collectively form the septal nuclei. Consequently, the subcallosal area, together with a small strip of cortex located adjacent to the lamina terminalis, the paraterminal gyrus, is commonly called the septal area. The septal nuclei are medially adjacent to the nucleus accumbens and continuous with sheets of neuronal cell bodies that extend into the septum pellucidum. The latter structure extends, in general, from the fornix to the inner surface of the corpus callosum. It forms the medial wall of the anterior horns and a small part of the bodies of the lateral ventricles. In general, the septal nuclei have complex interconnections with hippocampal, amygdaloid, and other limbic structures.

Temporal Lobe Lesions

Injury to the temporal lobe, especially bilateral damage, almost always involves the hippocampus and amygdala. Deficits most directly linked to trauma to these structures include profound changes in eating and sexual behavior, a decrease in aggression levels, and deficits in memory function. Regarding memory deficits, the patient may demonstrate a loss of recent memory or show an inability to acquire new memory (learn new tasks) while memory of events that took place in the distant past remains intact.

Vasculature of the Hippocampus and Amygdala

The blood supply to the hippocampal formation and amygdaloid complex is primarily via the anterior choroidal artery. This vessel arises from the internal carotid, passes along the medial edge of the temporal horn, and sends branches into the hippocampus and amygdala. It also serves the tail of the caudate, the choroid plexus of the temporal horn, the inferior regions of the lenticular nucleus, and the optic tract. The cortex of the uncus and that of the parahippocampal gyrus are served by superficial branches of the middle cerebral and posterior cerebral arteries, respectively.