Modern Psychology: C. G. Jung’s Lectures at the ETH Zürich, 1933-1941
The purified and nourished serpent in alchemy is the mercurial serpent, the Ouroboros, which is connected with the round thing.
It is one of the basic symbols in alchemy and refers to Mercury, not as ordinary mercury or quicksilver, but to the god or spirit Mercury.
The serpent is a Gnostic symbol for the spinal cord and the basal ganglia, because a snake is mainly backbone.
Snakes are weird and strange, and on account of this they have been used as a symbol for the unconscious since olden times.
If the unconscious can be localized anywhere it is in the basal ganglia, and it has the same uncanny character.
The snake really represents the vegetative psyche, the basis of the instincts, if one may express it in that way.
It is here (and in this place in the human being) that the greatest secret is to be found, the panacea, the universal medicine; and, according to the text, fortunate indeed is the man who finds it. ~Carl Jung, ETH, Lecture XI, Page 97.