The precious cauldron (yu ting)is a cavity in the centre of the brain (between and behind the eyes)and is the seat of (essential)nature, that is the original cavity of spirit(yuan shen shih or the ancestral cavity, tsu ch'iao); its left and right sides are linked with the pupils of the eyes by two (psychic) channels; and it is also connected with the heart.1 Hence it is said that essential nature is (in)the heart which manifests through the two eyes.

It is also said that essential nature is in the precious cauldron which originally did not exist.It is only when true vitality develops and unites with essential nature to become one whole that the latter is called the precious cauldron.
It is also said that about I3 inches under the navel is the cavity of real vitality (chen ch'i hsueh or lower tant'ien)which lies between the front and back of the lowerabdomen in proportion of seven to three and which is also called the golden stove (chin lu),the seat of (eternal)life.

Hence it is said that life is the generative force which de-velops in the genital organ.The golden stove originally did not exist but comes into being when the generative force de-velops and vitality manifests.

When the blood reaches the stove it changes into negative generative force which originates from this centre.
Saliva flows from two channels under the tongue.If it enters the jen mo channel (of function)it goes down into the lower tan t'ien centre (under the navel)to change into negative generative force, but a competent master should be sought to teach the proper way of swallowing saliva; other wise it will enter the stomach and intestines to be discharged as waste.

Source

The boy Buddha rising up from lotus. Crimson and gilded wood, Trần-Hồ dynasty, Vietnam, 14th-15th century.