Define the location and function of the following structures:
$(1)$ Epithalamus
$(2)$ Rod cells

(N/A) $(1)$ Location: It is a non-nervous part of the diencephalon that is fused with the piamater to form the anterior choroid plexus.
Function: It is involved in the secretion of cerebrospinal fluid and helps in the regulation of the endocrine system.
$(2)$ Location: These are rod-shaped photoreceptor cells found in the retina of the eye.
Function: They are specialized for vision in dim light (scotopic vision) and contain the pigment rhodopsin.