**Core Concept:** The hippocampus is an important region in the medial temporal lobe involved in learning, memory, and emotional regulation. The hypothalamus is a region within the diencephalon that controls various endocrine and autonomic functions. The connection between hippocampus and hypothalamus is essential for proper functioning of these structures.

**Why the Correct Answer is Right:** The hippocampus and hypothalamus are interconnected via the fornix, a white matter tract that consists of a bundle of axons. The fornix originates from the hippocampus and projects to the hypothalamus, allowing for communication between these two regions.

**Why Each Wrong Option is Incorrect:**

A. **Structures of the limbic system:** While the limbic system contributes to emotion and memory, it is not the correct answer because the fornix does not connect the hippocampus to the limbic system. The limbic system includes structures like amygdala, septum, and thalamus.

B. **The mammillary bodies:** These are part of the hypothalamus, not the correct answer. The mammillary bodies are involved in memory consolidation but not the direct connection between the hippocampus and hypothalamus.

C. **The mammillary bodies and the thalamus:** This option combines two structures, one in the hypothalamus (mammillary bodies) and one in the diencephalon (thalamus). The fornix connects the hippocampus to the hypothalamus directly, bypassing the thalamus.

D. **The cerebral cortex:** The cerebral cortex is part of the neocortex and is not involved in the direct communication between the hippocampus and hypothalamus.

**Clinical Pearl:** The fornix is essential for the integration of emotional and cognitive processing, ensuring optimal functioning of the hippocampus and hypothalamus. This connection is crucial for memory consolidation and regulation of the autonomic nervous system, including the HPA (hypothalamus-pituitary-adrenal) axis.

**Correct Answer:** C. **The mammillary bodies and the thalamus:** The mammillary bodies, part of the hypothalamus, receive information from the hippocampus via the fornix and relay this information to the thalamus, which then sends the information to the cerebral cortex for further processing and integration.