Li Daochun's (13th century) "Zhonghe ji" (The Harmony of the Center), ch. 2.
In the methods of the Way there are 3,600 gates: everyone clings to one of them as one's own root.
You should know that the tiny Opening of the Mysterious Barrier is not among those 3,600 gates.
The main point is that this Opening is the utterly mysterious and utterly wondrous Barrier. Here life and death part from one another, and here the saintly and the ordinary separate from one another. This is the secret transmitted from one Patriarch to the next one since ancient times until the present day; it is not something that can be known by means of random conjectures.
Among the students of the later generations who have not encountered a true master, some have thought that this Opening is the mouth and the nose; some that it is the point between the eyebrows; some that it is the fontanel; some that it is the cavity of the Hundred Convergences; some that it is the throat; some that it is the Spinal Handle; some that it is the Caudal Funnel; some that it is the opening of the heart; some that it is the Yellow Court; and some that it is the Cinnabar Field, the Origin of the Barrier, or the Ocean of Breath.
None of the above is the One Opening of the Mysterious Barrier. The Mysterious Barrier has no form and no image: how could it have a position? It is not form and it is not emptiness: how could it have a place? If it could be seen through its place or its position, it would be something provided with form and image, and we could not call it Mysterious Barrier.
This Opening does not not stick to the illusory body, and yet it is not separated from the illusory body. It does not stick to the illusory body because it is not a common thing provided with a form; it is not separated from the illusory body because it cannot be sought outside the body. In other words, it is neither something outside the body nor something inside the body. Therefore it can only exist in what is neither inside nor outside. There is a special celestial mechanism in this.
This Opening has a shape similar to Penglai:
outside it is small, inside it is large,
and its depth cannot be fathomed.
It is not round and it is not square;
within it, "the black and the white tally with each other,"
and darkness and light pervade one another.
Its gate is fifty feet high and four feet wide,
and has two panels:
once they open, once they close.
On its left coils a green dragon,
on its right is couched a white tiger,
above flies a vermilion sparrow,
below rests a black turtle.
Vague and indistinct! Dim and obscure!
A True Man lives inside it:
his name is Spirit of the Valley,
his appellation is Living a Long Life.
At daytime, he eats a broth of millet;
at night, he drinks the liquor of the boundless.
Sometimes he sings, clear and peaceful;
sometimes he is motionless, and keeps his mouth closed.
When he exhales, the gate of the Opening is wide open,
when he inhales, the gate of the Opening is firmly shut.