Similar to the brainstem, three different regions comprise the diencephalon of the adult human brain: the epithalamus, the thalamus, and the hypothalamus. Within the epithalamus is a group of capillaries collectively called a choroid plexus that manufactures cerebrospinal fluid. The epithalamus is also the site of the pineal gland, a constituent of the endocrine system that secretes the hormone melatonin, which appears to play an important role in regulating the sleep cycle and other cyclical behaviors. In mammals, the pineal gland is positioned near the center of the brain, but in some lower vertebrates the gland is located in closer proximity to the surface of the brain. Modern studies suggest that the pineal gland was possibly an evolutionary predecessor of the eye, a far cry from the belief of the seventeenth century philosopher René Descartes, who suggested it accommodated the soul.