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 possessed 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 A.D. According to Galen,
Herophilus (325–280 B.C.) had already considered the structure as a kind of valve that partitioned the brain chambers, particularly for the flow of vital spirits (
pneuma). Specifically, Herophilus believed that the structure was a
tap that controlled the movement of vital spirits from the middle (now identified as 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 of it as a valve for two basic reasons: it is located outside of the brain tissue and it does not move on its own.'' 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
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 presence of a pineal body was already discovered by German zoologist Franz Leydig in 1872, in European lizards. Leydig called them the "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, 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 hormone from the pineal gland might be useful in treating skin diseases, isolated it and named it
melatonin in 1958. The substance did not prove to be helpful as intended, but its discovery helped solve several mysteries, such as why removing a rat's pineal gland accelerated its 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 to be discovered. == Society and culture ==