The planet Mercury has multiple functions and as a messenger is involved in all communication systems within the body including correct nerve function,  hormone delivery and anything that requires transportation from one area to another via the tubes of the body.  Mercury governs the eyes, arms, mind/brain and shares many functions with other planets, especially it’s higher octave found in Uranus.  With Venus it rules the thyroid gland and with Mars it assists in the breakdown and delivery of nutrients from the digestive process.  Mercury delivers essential nutrients to the cells for their proper functioning and is concerned with the many facets of the digestive system through its rulership of Virgo and the lungs through Gemini.

The relationship between Mercury, as Hermes Trismegistus, and Neptune is through their rulership of the pineal gland and concerned with the transmutation of energy, specifically sexual energy.