the pineal gland that regulates circadian rhythms. in humans and other mammals, the light signals necessary to set circadian rhythms are sent from the eye through the retinohypothalamic system to the suprachiasmatic nuclei (scn) and the pineal gland. the fossilized skulls of many extinct vertebrates have a pineal foramen (opening), which in some cases is larger than that of any living vertebrate. although fossils seldom preserve deep-brain soft anatomy, the brain of the russian fossil bird cerebavis cenomanica from melovatka, about 90 million years old, shows a relatively large parietal eye and pineal gland. history diagram of the operation of the pineal gland for descartes in the treatise of man (figure published in the edition of 1664) the secretory activity of the pineal gland is only partially understood. its location deep in the brain suggested to philosophers throughout history that it possesses particular importance. this combination led to its being regarded as a "mystery" gland with mystical, metaphysical, and occult theories surrounding its perceived functions. the earliest recorded description of the pineal gland is from the greek physician galen in the 2nd century c.e. according to galen herophilus (325–280 b.c.e.) had already considered the structure as a kind of valve that partitioned the brain chambers, particularly for the flow of vital spirits (pneuma). specifically, the ancient greeks believed that the structure maintains the movement vital spirits from the middle (now identified to the third) ventricle to the one in the parencephalis (fourth ventricle). galen described the pineal gland in de usu partium corporis humani, libri vii (on the usefulness of parts of the body, part 8) and de anatomicis administrationibus, libri ix (on anatomical procedures, part 9). he introduced the name κωνάριο (konario, often latinised as conarium) that means cone, as in pinecone, in de usu partium corporis humani. he correctly located the gland as directly lying behind the third ventricle. he argued against the prevailing concept as a valve for two basic reasons: it is located outside of the brain tissue and it does not move on its own. galen instead identified the valve as a worm-like structure in the cerebellum (later called vermiform epiphysis, known today as the vermis cerebelli or cerebellar vermis). from his study on the blood vessels surrounding the pineal gland he discovered the great vein of the cerebellum, later called the vein of galen. he could not establish any functional role of the pineal gland and regarded it as a structural support for the cerebral veins. seventeenth-century philosopher and scientist rené descartes discussed the pineal gland both in his first book, the treatise of man (written before 1637, but only published posthumously 1662/1664), and in his last book, the passions of the soul (1649) and he regarded it as "the principal seat of the soul and the place in which all our thoughts are formed". in the treatise of man, he described conceptual models of man, namely creatures created by god, which consist of two ingredients, a body and a soul. in the passions, he split man up into a body and a soul and emphasized that the soul is joined to the whole body by "a certain very small gland situated in the middle of the brain's substance and suspended above the passage through which the spirits in the brain's anterior cavities communicate with those in its posterior cavities". descartes gave importance to the structure as it was the only unpaired component of the brain. the latin name pinealis became popular in the 17th century. for example, english physician thomas willis described glandula pinealis book, cerebri anatome cui accessit nervorum descriptio et usus (1664). willis criticised descartes' concept, remarking: "we can scarce believe this to be the seat of the soul, or its chief faculties to arise from it; because animals, which seem to be almost quite destitute of imagination, memory, and other superior powers of the soul, have this glandula or kernel large and fair enough." walter baldwin spencer at the university of oxford gave the first description of the pineal gland in lizards. in 1886, he described an eye-like structure, which he called the pineal eye or parietal eye, that were associated with the parietal foramen and the pineal stalk. the main pineal body was already discovered by german zoologist franz leydig in 1872 from european lizards. leydig called them frontal organ (german stirnorgan). in 1918, swedish zoologist nils holmgren described the parietal eye in frogs and dogfish. he discovered that the parietal eyes were made up of sensory cells similar to the cone cells of the retina, and suggested that it was a primitive light-sensor organ (photoreceptor). the pineal gland was originally believed to be a "vestigial remnant" of a larger organ. in 1917, it was known that extract of cow pineals lightened frog skin. dermatology professor aaron b. lerner and colleagues at yale university, hoping that a substance from the pineal might be useful in treating skin diseases, isolated and named the hormone melatonin in 1958. the substance did not prove to be helpful as intended, but its discovery helped solve several mysteries such as why removing the rat's pineal accelerated ovary growth, why keeping rats in constant light decreased the weight of their pineals, and why pinealectomy and constant light affect ovary growth to an equal extent; this knowledge gave a boost to the then new field of chronobiology. of the endocrine organs, the function of the pineal gland was the last discovered. society and culture the notion of a "pineal-eye" is central to the philosophy of the french writer georges bataille, which is analyzed at length by literary scholar denis hollier in his study against architecture. in this work hollier discusses how bataille uses the concept of a "pineal-eye" as a reference to a blind-spot in western rationality, and an organ of excess and delirium. this conceptual device is explicit in his surrealist texts, the jesuve and the pineal eye. in the late 19th century madame blavatsky (who founded theosophy) identified the pineal gland with the hindu concept of the third eye, or the ajna chakra. this association is still popular today. in the short story "from beyond" by h. p. lovecraft, a scientist creates an electronic device that emits a resonance wave, which stimulates an affected person's pineal gland, thereby allowing them to perceive planes of existence outside the scope of accepted reality, a translucent, alien environment that overlaps our own recognized reality. it was adapted as a film of the same name in 1986. the 2013 horror film banshee chapter is heavily influenced by this short story. additional images the pineal body is labeled in these images. mesal aspect of a brain sectioned in the median sagittal plane dissection showing the ventricles of the brain hind- and mid-brains; antero-lateral view median sagittal section of brain pineal gland brainstem; posterior view