<P> The results of various scientific research in evolutionary biology, comparative neuroanatomy and neurophysiology, have explained the phylogeny of the pineal gland in different vertebrate species . From the point of view of biological evolution, the pineal gland represents a kind of atrophied photoreceptor . In the epithalamus of some species of amphibians and reptiles, it is linked to a light - sensing organ, known as the parietal eye, which is also called the pineal eye or third eye . </P> <P> René Descartes believed the pineal gland to be the "principal seat of the soul". Academic philosophy among his contemporaries considered the pineal gland as a neuroanatomical structure without special metaphysical qualities; science studied it as one endocrine gland among many . However, the pineal gland continues to have an exalted status in the realm of pseudoscience . </P> <P> The pineal gland is a midline brain structure that is unpaired . It takes its name from its pine - cone shape . The gland is reddish - gray and about the size of a grain of rice (5--8 mm) in humans . The pineal gland, also called the pineal body, is part of the epithalamus, and lies between the laterally positioned thalamic bodies and behind the habenular commissure . It is located in the quadrigeminal cistern near to the corpora quadrigemina . It is also located behind the third ventricle and is bathed in cerebrospinal fluid supplied through a small pineal recess of the third ventricle which projects into the stalk of the gland . </P> <P> Unlike most of the mammalian brain, the pineal gland is not isolated from the body by the blood--brain barrier system; it has profuse blood flow, second only to the kidney, supplied from the choroidal branches of the posterior cerebral artery . </P>

The can be found as part of the epithalamus near the superior colliculi of the midbrain