Brief History of the Discovery of the Pineal Gland (Epiphysis)
The pineal gland is about the size of a grain of rice. Therefore its initial discovery was difficult and late in coming. Galen (2nd century) was probably the first to describe it in the West. He thought it might be a valve to regulate the flow of thought from the lateral ventricles–cavities on each side–of the brain.  Rene Descartes, the French philosopher, who made a number of rather remarkable scientific discoveries wrote about the gland 1500 years after Galen. In Descartes opinion the pineal was the “seat of the soul”. He also postulated a direct connection between the eyes and the pineal by means of “strings” in the brain. Also that the gland acted as an interpreter, indeed the chief interpreter of vision. Not only did the gland operate as an interpreter but it also directed the muscles to respond to objects in the visual field. This was done, Descartes believed, through the flow of humours passing through hollow tubes between the gland and the muscles. 
The Optic Third Eye Compared to the Endocrinal Pineal Gland
The three animals previously mentioned (Western Fence Lizard, Pacific Tree Frog, and Sea Lamprey) are to be considered now for their contribution to the research being done on the optic importance of the pineal body.
Since the first discovery, right on down to present findings, there has been the question of the pineal’s relation to light. How romantic to think of a functional third eye pointed skyward for the ultimate in ground protection! Other obvious benefits are associated with the having of such a receptor.
In the Western Fence Lizard (S. occidentalis) the pineal and the parietal third eye are connected by means of the parietal nerve. The epiphysis is located above the cortex and under the bone of the skull. Under high magnification one sees the ultrastructures of the cornea, lens, and retina. The cornea is composed of an inner, highly fibrous layer and an epidermal layer. The cornea is fused with the lens, a palisade of elongate, cylindrical cells whose nuclei lie at their basal ends. A fibrous capsule encloses the eye and attaches it to the skin. The parietal nerve leaves the retina, passes through the capsule, and courses posteriorly under the roof of the cranium and then ventrally to the epiphysis and brain. 
We know that the parietal eye is functional because there are changes in electrical activity, which can be recorded from the retina (ERG) or parietal eye nerve when light to the eye is turned off or on. It is also interesting that a deficiency of vitamin A causes a breakdown in the outer segments of third eye receptors in S. occidentalis. Let it be said now that the third eye contains a light sensitive substance (or perhaps two substances) since it reacts differently to short and long wave lengths of light.
Serotonin, LSD, and the Epiphysis (Third Eye)
In the last section we described some of the physiology of serotonin, the pineal gland and its synthesis of the hormones serotonin and melatonin. Serotonin is a normal, necessary chemical transmitter of electrical impulses across the synapses (the gaps between nerve cell bodies). It is intriguing to find that certain hallucinogens have the same chemical skeletons as serotonin.  This really doesn’t surprise neurologicians, for the fact of psychedelically induced psychosis has been known.
As mentioned, serotonin is one of the four main neurohumors or neurotransmitters in higher vertebrate nervous systems. I have mentioned the location of serotonin production and note here that the serotonin is transported via the bloodstream to the nerve cells throughout the body, but most especially in the neurons of the brain. Here they accumulate in the their minutest molecular form. The molecule serotonin is utilized by the nerve cells for the complete execution of electrical impulses across the synaptic gap (which is the micro-gap between every connection of every nerve cell in the entire nervous system). The impulses comes along the nerve cell going through the electro-chemical processes with the ionic forms of calcium and potassium (the two vitals of the nervous system) until they reach the terminal end of the cell’s dendrites. Upon reaching the end of the electrical impulse is translated into the neurochemical serotonin. This is then “squeezed” out into intercellular space only to connect and meet the other side which is the beginning of the next nerve soma (lining of the nerve cell). 
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