The pineal gland is a small gland, part of the endocrine system, which is found in the center of the brain, between the two hemispheres, near the thalamus. It produces melatonin, a derivative of serotonin, which regulates the wake/sleep pattern, including adjusting for the change of light levels during different seasons. It is very small (about 5–8 mm long in anyone over the age of two) and gets its name from its resemblance to a pine cone. It is one of the few glands in the skull that will show up on x-rays as it is often calcified.
Because the cells of the pineal gland resemble those that sense light in the retina, many biologists believe the gland and the retina share a common ancestry. For this reason, and the gland's affinity for light (constant light or darkness impedes it's function) it is often called the "third eye".
Because the gland calcifies in most species, it is often the only gland that shows up in fossils.