The nucleus mammillaris medialis is the larger of the two wart nuclei, such a pair of hay nuclei. and one on the left. page!
The nucleus receives connections from the hippocampus complex via fornix postcommissurale, transmits signals to the anterior thalamus nucleus via the tractus mammillo-thalamicus and has round-trip connection with the nucleus tegmentalis ventralis.
From the anterior thalamus nucleus, the information is passed on to the cinguli gyrus and from there via the cingulal association pathway, e.g. back to the hippocampus complex (Papez's circuit).
The nucleus mammillaris medialis with its connections is part of the network that is actively moving in, and investigating the outside world. The coupling system is of paramount importance for the spatial working memory and episodic memory required for the movement to be meaningful and repeatable.
The two mammomm nuclei are often described as crucial for learning and memory storage, a view that is rooted in observations of patients with Korsakoff's disease/syndrome, which among other things involves severe memory loss (amnesia). The perception is based on the small hemorrhages and degenerations that are always found in the medial mammillary nucleus of the deceased Korsakoff patient.
A more modern theory is that amnesia is primarily due to anterior, dorsomedial, and median thalamus nuclei, all of which receive some signal inflow from the medial mammillary nucleus. The latter would then, as long as it is undamaged, be able to mask/support disabled thalamus nuclei.
Vann S.D. and Aggleton J.P. (2004) The mammillary bodies: two memory systems in one? Nature Reviews Neuroscience 5, 35-44.